To allow overriding of the boot stack size from the Mbed configuration
system, consistently use MBED_CONF_TARGET_BOOT_STACK_SIZE rather than
MBED_BOOT_STACK_SIZE.

Fixes #10319

--legacyalign, --no_legacyalign are deprecated from ARMC6 compiler, in order to
remove deprecated flags all linker files (GCC and IAR as well to have uniformity)
should strictly align to 8-byte boundary

* Make memory sections configurable in linker files
* Dynamically determine vector location in flash for NVIC relocation
* Advertise bootloader support in targets.json

* Off by one error in the linker scripts reserved one word too little for the vector table
* Re-apply uvisor changes to emlib. To allow uvisor to run, we should make accesses to the romtable through uvisor's secure read gateway
* Copypasta in target name (EFM32PG12, not EFR32PG12)
* Copypasta in the pin definitions (thanks @akselsm)
* Forgot to update PortName for extra ports on MG/PG12