Parametr board_build.cmsis.startup_file doesn't work with absolute Path.

[Ampermetr123]

If we have configuration like this

platform = ststm32
board = nucleo_h745zi_q
framework = cmsis
board_build.ldscript = $PROJECT_DIR/system/h745cm7_flash.ld
board_build.cmsis.system_file = $PROJECT_DIR/system/system.c
board_build.cmsis.startup_file = $PROJECT_DIR/system/startup.s  

the last line won't work. None default or custom startup file is added to build tree.
Now it supports only specific startup filenames placed in platform folder.

Even we could add the own startup-file manually with build_src_filter for main application,
the apps in test folder are abandoned without any startup file.

It would be very convenient to have the opportunity to specify custom startup file.
In other words, to add support of abs path for board_build.cmsis.startup_file.