CMake: Fix STM target build failures

Ensure the linker and startup files listed match
with the correct letter case as some host systems
are case sensitive.

Author: hugueskamba

targets/TARGET_STM/TARGET_STM32F4/TARGET_STM32F407xE/TOOLCHAIN_ARM/startup_STM32F407xx.S renamed to targets/TARGET_STM/TARGET_STM32F4/TARGET_STM32F407xE/TOOLCHAIN_ARM/startup_stm32f407xx.S

@@ -3,10 +3,10 @@
 
 if(${MBED_TOOLCHAIN} STREQUAL "GCC_ARM")
     set(STARTUP_FILE TOOLCHAIN_GCC_ARM/startup_stm32h747xx.S)
-    set(LINKER_FILE TOOLCHAIN_GCC_ARM/STM32H747xI_CM4.ld)
+    set(LINKER_FILE TOOLCHAIN_GCC_ARM/stm32h747xI_CM4.ld)
 elseif(${MBED_TOOLCHAIN} STREQUAL "ARM")
     set(STARTUP_FILE TOOLCHAIN_ARM/startup_stm32h747xx.S)
-    set(LINKER_FILE TOOLCHAIN_ARM/stm32h747xi-cm4.sct)
+    set(LINKER_FILE TOOLCHAIN_ARM/stm32h747xI_CM4.sct)
 endif()
 
 add_library(mbed-stm32h747xi-cm4 INTERFACE)