Add pxe command

Add pxe command, which is intended to mimic PXELINUX functionality.
'pxe get' uses tftp to retrieve a file based on UUID, MAC address or IP
address. 'pxe boot' interprets the contents of PXELINUX config like file
to boot using a specific initrd, kernel and kernel command line.

This patch also adds a README.pxe file - see it for more details on the
pxe command.

Signed-off-by: Jason Hobbs <jason.hobbs@calxeda.com>