[csolution] does not check if the specified linker script file is present

[ReinhardKeil]

Describe the bug
csolution does not check if the linker script file is present. Instead an error at CMake build level is generated that is misleading.

ninja: error: 'C:/w/csolution-examples/CubeMX/RTE/Device/STM32U585AIIx/ac6_linker_script.sct.srcx', needed by 'C:/w/csolution-examples/CubeMX/out/CubeMX/MyBoard_ROM/Debug/CubeMX.axf', missing and no known rule to make it

To Reproduce

Use csolution-examples CubeMX and change *.cproject.yml the linker script file name, i.e.

  linker:
    - script: RTE/Device/STM32U585AIIx/ac6_linker_script.sct.srcx
    - regions: $regions_header$
      for-compiler: AC6

Expected behavior
Missing source files are flagged by csolution. Implement a similar behavour for linker script file.

Environment (please complete the following information):

• CMSIS-Toolbox 2.7.0
• Windows