x86: get_bios_ebda_length()

Add a wrapper routine that tells us the length of the EBDA if it is
present.  This guy also ensures that the returned length doesn't let the
EBDA run past the 640KiB mark.

Signed-off-by: Mike Waychison <mikew@google.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>