[M] Change in coreboot[main]: arch/riscv: Factor out common romstage code

[Maximilian Brune (Code Review)]

Attention is currently required from: Arthur Heymans, Philipp Hug, Ron Minnich.

Maximilian Brune has posted comments on this change by Maximilian Brune. ( https://review.coreboot.org/c/coreboot/+/83098?usp=email )

Change subject: arch/riscv: Factor out common romstage code
......................................................................

Patch Set 4:

(1 comment)

File src/arch/riscv/romstage.c:

https://review.coreboot.org/c/coreboot/+/83098/comment/e670647d_1d8bbea5?usp... :
PS2, Line 10: _weak void platform_romstage_main(void) { /* no-op, for bring-up */ }
> Would it not be better if bring-up code explicitly adds an empty function rather than putting a weak […]

I don't really care, but I am not sure how common it is to not have one. I guess usually only emulation targets have that case.
Ok then I changed it.