Fix Zephyr (gcc) build issues

[rustylocker]

I'm using libosdp-rs in Zephyr building with GCC (defined __BARE_METAL__, __ZEPHYR__ ).
And I faced the following issues:

• Duplicated symbol gettimeofday (linker issue) and
• __noreturn undefined

[sidcha]

__ZEPHYR__ and __BARE_METAL__ are mutually exclusive as zephyr is an RTOS. We need to fix the real problem which causes __BARE_METAL__ to be enabled when you build for zephyr. The fix would be 2 parts:

• Fix why you get __BARE_METAL__ defined
• Fix platform detection macros to include __ZEPHYR__ so you don't get a duplicate gettimeofday definition (like it is done for linux for example).

I can take a look at this a bit later. But please feel free to investigate it yourself.

[rustylocker]

In my environment __BARE_METAL__ is defined in build.rs because CARGO_CFG_TARGET_OS is none. Up to the documentation the latter is perfectly normal for embedded devices.

If I define __ZEPHYR__ forcefully (without __BARE_METAL__) the compiler can't find <zephyr/toolchain>.
And when I add zephyr/include to the include directories in build.rs I get the preprocessor error "processor architecture not supported".

warning: libosdp-sys@3.1.0: In file included from /workdir/zephyr/include/zephyr/toolchain.h:52,
warning: libosdp-sys@3.1.0:                  from vendor/utils/include/utils/utils.h:152,
warning: libosdp-sys@3.1.0:                  from vendor/utils/include/utils/slab.h:14,
warning: libosdp-sys@3.1.0:                  from vendor/utils/src/slab.c:11:
warning: libosdp-sys@3.1.0: /workdir/zephyr/include/zephyr/toolchain/gcc.h:622:2: error: #error processor architecture not supported
warning: libosdp-sys@3.1.0:   622 | #error processor architecture not supported
warning: libosdp-sys@3.1.0:       |  ^~~~~
error: failed to run custom build command for `libosdp-sys v3.1.0

Caused by:
  process didn't exit successfully: ... (exit status: 1)
  --- stdout
  cargo:warning=Forced building for Zephyr target
  OPT_LEVEL = Some(z)
  OUT_DIR = Some(...)
  TARGET = Some(thumbv7em-none-eabi)
  CARGO_ENCODED_RUSTFLAGS = Some()
  HOST = Some(x86_64-unknown-linux-gnu)
  cargo:rerun-if-env-changed=CC_thumbv7em-none-eabi
  CC_thumbv7em-none-eabi = None
  cargo:rerun-if-env-changed=CC_thumbv7em_none_eabi
  CC_thumbv7em_none_eabi = Some(/opt/toolchains/zephyr-sdk-0.17.1/arm-zephyr-eabi/bin/arm-zephyr-eabi-gcc)
  RUSTC_WRAPPER = None
  cargo:rerun-if-env-changed=CC_ENABLE_DEBUG_OUTPUT
  cargo:rerun-if-env-changed=CRATE_CC_NO_DEFAULTS
  CRATE_CC_NO_DEFAULTS = None
  DEBUG = Some(false)
  CARGO_CFG_TARGET_FEATURE = None
  cargo:rerun-if-env-changed=CFLAGS
  CFLAGS = None
  cargo:rerun-if-env-changed=TARGET_CFLAGS
  TARGET_CFLAGS = None
  cargo:rerun-if-env-changed=CFLAGS_thumbv7em_none_eabi
  CFLAGS_thumbv7em_none_eabi = None
  cargo:rerun-if-env-changed=CFLAGS_thumbv7em-none-eabi
  CFLAGS_thumbv7em-none-eabi = None
  cargo:warning=In file included from /workdir/zephyr/include/zephyr/toolchain.h:52,
  cargo:warning=                 from vendor/utils/include/utils/utils.h:152,
  cargo:warning=                 from vendor/utils/include/utils/slab.h:14,
  cargo:warning=                 from vendor/utils/src/slab.c:11:
  cargo:warning=/workdir/zephyr/include/zephyr/toolchain/gcc.h:622:2: error: #error processor architecture not supported
  cargo:warning=  622 | #error processor architecture not supported
  cargo:warning=      |  ^~~~~

  --- stderr

  error occurred in cc-rs: command did not execute successfully (status code exit status: 1): LC_ALL="C" "/opt/toolchains/zephyr-sdk-0.17.1/arm-zephyr-eabi/bin/arm-zephyr-eabi-gcc" "-Os" "-ffunction-sections" "-fdata-sections" "-mthumb" "-march=armv7e-m" "-I" "vendor/src" "-I" "vendor/include" "-I" "vendor/utils/include" "-I" ... "-I" "/workdir/zephyr/include" "-Werror" "-Wall" "-Wextra" "-fshort-enums" "-D __ZEPHYR__=1" "-o" ... "-c" "vendor/utils/src/slab.c"

Do you have an idea how to solve this?