[custom boards] add default weak TargetBSP_Init function

[jeromecoutant]

Description of defect

Discussion started in #13022

Goal is to be able to create a custom board which inherit from another board

Proposition from @kjbracey-arm

Add in a generic CMSIS startup.S, something like:

BL SystemInit
BL UserEarlyInit   // default WEAK empty implementtion
BL __main

@AGlass0fMilk
@MarceloSalazar
@0xc0170

Target(s) affected by this defect ?

NA
All

Toolchain(s) (name and version) displaying this defect ?

NA
All

What version of Mbed-os are you using (tag or sha) ?

mbed-os-6.0.0

What version(s) of tools are you using. List all that apply (E.g. mbed-cli)

NA

How is this defect reproduced ?

NA

[jeromecoutant]

Solution should take into account #13130
Solution should be consistent for all targets.

@kjbracey-arm

[kjbracey]

That "weak" thing wasn't my ideal suggestion - it doesn't quite fit with the general inheritance mechanism. I preferred my "alternate init function" version.

[jeromecoutant]

That "weak" thing wasn't my ideal suggestion - it doesn't quite fit with the general inheritance mechanism. I preferred my "alternate init function" version.

Maybe I didn't fully understood, but I preferred the weak approach as the goal is to add some additional init, not to replace the current SystemInit ?

[ciarmcom]

Thank you for raising this detailed GitHub issue. I am now notifying our internal issue triagers.
Internal Jira reference: https://jira.arm.com/browse/IOTOSM-2287