[coreboot] printing TSC in printk

[ron minnich]

Here's a simple proposal.

Define an option
PRINTK_TSC

What it does: each time printk would print a newline, it will instead
print this:
(16 hex digits of TSC)\n

Here's another simpler option:

Define a new format letter, T, such that %T as a format means "time".

first option allows comprehensive timing, but it will slow things down
a bit. Second option allows us to completely tailor the printing of
time, but you have to explicitly add %T when you want time printed.

Comments?

ron