libpayload: fix compiler warning for first_cmos_entry()

libpayload: fix compiler warning for first_cmos_entry()

The 'name' argument to lookup_cmos_entry() is declared to be 'char *'
but we pass an empty string ("") which is 'const char[]' so the compiler
legitimatly warns about discarded qualifiers here. Fix this by passing
NULL as 'name'.

Minor nitpick: The NULL test in lookup_cmos_entry() is superfluous as our
implementation of strnlen() can handle NULL pointers gracefully. But for
an average C hacker it just doesn't feel right not to do so.

Change-Id: I592917d12d8fa840804c0d19e38b844427064fef
Signed-off-by: Mathias Krause <mathias.krause@secunet.com>
Reviewed-on: http://review.coreboot.org/651
Tested-by: build bot (Jenkins)
Reviewed-by: Stefan Reinauer <stefan.reinauer@coreboot.org>