[M] Change in coreboot[master]: Rename CBFS master header -> CBFS main header

[Julius Werner (Code Review)]

Attention is currently required from: Angel Pons, Jason Glenesk, Martin L Roth, Stefan Reinauer, Yu-Ping Wu.

Julius Werner has posted comments on this change. ( https://review.coreboot.org/c/coreboot/+/75707?usp=email )

Change subject: Rename CBFS master header -> CBFS main header
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Patch Set 7:

(1 comment)

Commit Message:

https://review.coreboot.org/c/coreboot/+/75707/comment/0e9ab1e6_69502039 :
PS5, Line 9: Calling it the main header is more descriptive of its pupose
           : and should therefore be preferred.
> Your opinion on this is noted. […]

How about calling it "CBFS legacy header" to indicate that it is old and deprecated (although for practical purposes of compatibility with old payloads, I'm not sure if we'll ever really want to remove it)?