[coreboot] libpayload: Add LAR walking support

[Jordan Crouse]

After all the other appetizers, here is the main course - LAR support
for libpayload.  This allows the payload to walk the lar and get
information about the headers.  I also added some functions modeled
on fopen() and friends, but I haven't tried those since I don't have
any reason to open the LAR.  The functions are loosely modeled on their
POSIX cousins.   Examples for how to make all of this work coming next
in the form of a coreinfo module.

Jordan