Without yet adding in the underlying code, extend the revision
checking code to return additional values for PandaBoard ES boards.
Much of the code was taken close to verbatim from U-Boot.

Signed-off-by: Robert P. J. Day <rpjday@crashcourse.ca>

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 compile tested, and confirmed that this code behaves identically to
the original panda configure and build for the three pandas i have:

 * Rev A2 (boots)
 * Rev EA3 (boots)
 * Rev B1 ES (no output)

so while there's still work to be done, this at least represents the
necessary infrastructure to start *recognizing* panda ES boards.