cmake: fix utterly cryptic error handling in the -DSPARSE=y build

[marc-hb]

Fixes commit 7a85ff768385 ("add sparse support")

The sparse build needs (at least) two things:

1. sparse to be in the PATH
2. the environment variable REAL_CC to be defined and to match the value expected by CMake

Fix error messages when either condition is wrong.

• New error message when 1. is not satisfied:

CMake Error at zephyr/cmake/compiler/gcc/target.cmake:12 (find_program):
  Could not find CMAKE_C_COMPILER using the following names: cgcc

• Previous error "message" when 1. is not satisfied:

CMake Error at zephyr/cmake/compiler/gcc/target.cmake:17 (message):
  C compiler
  ZSDK/xtensa-intel_s1000_zephyr-elf/bin/xtensa-intel_s1000_zephyr-elf-gcc
  not found - Please check your toolchain installation

(the reported location DOES exist!)

• New error message when 2. is not satisfied:

CMake Error at zephyr/cmake/compiler/gcc/target.cmake:25 (message):
  sparse's REAL_CC is undefined, it must be set to:
  zsdk/xtensa-intel_s1000_zephyr-elf/bin/xtensa-intel_s1000_zephyr-elf-gcc

• Previous error "message" when 2. is not satisfied:

modules/hal/xtensa/include/xtensa/config/core.h:54:10:
    error:unable to open 'core-isa.h'`

Also: stop using the same name REAL_CC for both the internal CMake variable and the external sparse parameter; they're two different things so name them differently. Environment variable messes are complicated enough.

Signed-off-by: Marc Herbert [email protected]

[marc-hb]

• No idea why the "operation was cancelled" in the middle of test run https://github.com/zephyrproject-rtos/zephyr/actions/runs/3086675604/jobs/4991288674. Clearly unrelated.

• Just for the record this was found while enabling sparse in SOF CI: https://github.com/thesofproject/sof/pull/6311

• cc: @lucvoo (https://github.com/lucvoo/sparse), @keqiaozhang, @greg-intel

[marc-hb]

Thanks for patching something that is basically broken in its initial proposal, https://github.com/zephyrproject-rtos/zephyr/pull/43776#pullrequestreview-1023060702.

Just for the record, complete rework by @tejlmand in progress in

• #51212
• #52671