Fixes commit 7a85ff7683 ("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
```
Note the "not found" cross-compiler actually exists!
- New error message when 2. is not satisfied:
```
Kconfig header saved to 'ws/build-tgl/zephyr/include/generated/autoconf.h'
-- Found sparse: /home/user/sparse/cgcc
CMake Error at ws/zephyr/cmake/compiler/gcc/target.cmake:25 (message):
The only way to override its 'cc' default when cross-compiling with
sparse is unfortunately an environment variable. So you _must_ set
REAL_CC at both configuration time and build time 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 <marc.herbert@intel.com>