[coreboot] SeaBIOS boot menu key changed to ESC

[Kevin O'Connor]

In the past, F12 was the default SeaBIOS boot menu key.  This was
typically seen with a boot message of:

Press F12 for boot menu.

Do to several issues with the F12 key on both real and emulated
machines, the development branch of the SeaBIOS repository has now
changed the default key to the ESC key.  This means the prompt in the
future will show as:

Press ESC for boot menu.

If the old key is desired, one can still activate it by placing 0x86
in the CBFS file "/etc/boot-menu-key" (cbfstool build/coreboot.rom
add-int -i 0x86 -n etc/boot-menu-key) along with the old boot message
in "/etc/boot-menu-message".  The wiki documentation has futher
details.

-Kevin