staging: fpgaboot: clean up Makefile

This Makefile tries to set the DEBUG macro but it uses an unknown
Kconfig macro to do so. Since no code appears to even care about the
DEBUG macro this line can safely be removed.

Signed-off-by: Paul Bolle <pebolle@tiscali.nl>
Reviewed-by: Insop Song <insop.song@gainspeed.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>