[2.8] fix: FLASH overflow detection

[raphaelcoeffic]

Prior to this PR, FLASH overflow detection would fail if .text segment would be small enough, but the firmware would overflow once .data is added.

Summary of changes:

• explicitly locate .data segment in FLASH instead of using _sidata marker.

[raphaelcoeffic]

Seems now we have the CI build failing at the proper place:

[100%] Linking CXX executable ../../firmware.elf
/opt/gcc-arm-none-eabi-10-2020-q4-major/bin/../lib/gcc/arm-none-eabi/10.2.1/../../../../arm-none-eabi/bin/ld: ../../firmware.elf section `.data' will not fit in region `FLASH'
/opt/gcc-arm-none-eabi-10-2020-q4-major/bin/../lib/gcc/arm-none-eabi/10.2.1/../../../../arm-none-eabi/bin/ld: region `FLASH' overflowed by 624 bytes
collect2: error: ld returned 1 exit status

We should have had this before, which now works properly: region 'FLASH' overflowed by 624 bytes