SeaBIOS Makefile.inc: Remove build dir for uppermem option

The build directory got removed while my patch was in flight and I
didn't notice when I submitted it.

The uppermemory change was added in
commit 4d7d25f38a - http://review.coreboot.org/#/c/6364

The output directory was changed for everything else in
commit ab11a6a94c - http://review.coreboot.org/#/c/6460

Change-Id: Ib8311f694280d305e826adbb76e3e7b722b30e0f
Signed-off-by: Martin Roth <martin.roth@se-eng.com>
Reviewed-on: http://review.coreboot.org/7298
Tested-by: build bot (Jenkins)
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>
Reviewed-by: Patrick Georgi <pgeorgi@google.com>