board_build.cmsis.startup_file parameter can't handle absolute path

[Ampermetr123]

For CMSIS framework we can specify custom system-file with absolute or relative path:
board_build.cmsis.system_file = system_stm32h7xx_singlecore.c # relative using file from framework
board_build.cmsis.system_file = $PROJECT_DIR/system.c # absolute path - using file from project folder

But for startup file we can specify the relative path only
board_build.cmsis.startup_file = startup_stm32h7b3xx.s #ok
board_build.cmsis.startup_file = $PROJECT_DIR/startup.s #not work
It's not convenient, but easy to fix.