Update linker script templates to include stack

[c1728p9]

Update the linker script templates to include the ability to adjust the boot stack stack size.

This depends on the code change here:
ARMmbed/mbed-os#8039

[deepikabhavnani]

@c1728p9 - Is this document for Mbed 2 only?

This query is because in our source code linker files we set MBED_BOOT_STACK_SIZE as default to 0x400 but in document sample it is 0x1000.

I remember we set MBED_BOOT_STACK_SIZE for Mbed 2 in targets.json. Can we add that info and update example if bootstrap.md is for bothe Mbed 2 and Mbed 5.

[c1728p9]

Hi @deepikabhavnani, I set the default stack size to 4K to match the value in targets.json. The RTOS mbed_lib.json overrides this to 1K when present.

This linker script template is for both 2 and 5. Are you wanting this to be stated more explicitly? Can you elaborate on what you would like updated in bootstrap.md?

[AnotherButler]

@deepikabhavnani Is this code OK to merge, or does @c1728p9 need to make further changes?

[deepikabhavnani]

Needs changes

[c1728p9]

Made changes and rebased to latest.