[coreboot] Documenting the process of Timestamp collection during romstage

[Alex G.]

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The idea of collecting timestamps during romstage is awesome. the only
problem is that the code that does that is completely undocumented.
Some stuff you figure out by looking at the code.
Then there's timestamp_init(tsc_t), whose argument is read by the
mainboard from some mysterious registers.
Then there's also:

build/lib/timestamp.romstage.o: In function `timestamp_init':
/home/mrnuke/Sources/coreboot/src/lib/timestamp.c:40: undefined
reference to `cbmem_add'
build/lib/timestamp.romstage.o: In function `timestamp_add':
/home/mrnuke/Sources/coreboot/src/lib/timestamp.c:58: undefined
reference to `cbmem_find'


Can someone please explain the proper procedure of getting timestamp
collection running. I'll even sacrifice myself and add doxygen-style
documentation for the next poor fella trying to implement this.

Alex
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