Linker script doesn't get preprocessed

[kalle16lab]

Describe the bug
Our device has a custom bootloader and thus needs the ability to modify "MBED_APP_START" and "MBED_APP_SIZE" definitions in the linker script.

Using mbed studio 1.4.1 this worked when adding

"target.mbed_app_start" : "0x2e000",

to the mbed_app.json target overrides section.

but when using mbed-tools the linker script in cmake_build/<our custom target>/develop/GCC_ARM/mbed-os/targets/TARGET_<vendor>/TARGET_<family>/mbed-<family>.link_script.ld begins with

ENTRY(Reset_Handler)
MEMORY
{
  FLASH (rx) : ORIGIN = MBED_APP_START, LENGTH = MBED_APP_SIZE
  RAM_NVIC (rwx) : ORIGIN = MBED_RAM_START, LENGTH = 0x100
  RAM (rwx) : ORIGIN = (MBED_RAM_START + 0x100), LENGTH = (384K - (0x100))
}

Seemingly no preprocessing has been done. Adding

add_definitions("-DMBED_APP_START=0x0002e000")
add_definitions("-DMBED_APP_SIZE=0x000D2000")

to the CMakeListst.txt in app folder makes no diference.

To Reproduce
Have target with preproccesing directives in the linker script and attempt to compile using mbed-tools. Verify that the linker script hasn't been modified.

Expected behavior
Linker script preproccessing directives should be replaced with correct values.

Desktop (please complete the following information):

• OS: Windows 10

Mbed (please complete the following information):

• Mbed CLI 2 Version: 7.15.0

[kalle16lab]

This is a duplicate of #156