Makefile: Replace the way to test if a string is empty

The output of command below,
# i386-elf-nm build/cbfs/fallback/romstage_null.offenders | \
                     grep -q "" ; echo $?
has different result on MacOS, OS X Mavericks, which outputs 0.
On linux, it outputs 1.

I assume it is misleading to search an empty string in a empty
string. Change it to testing if the string is empty.

Change-Id: Ie4b8fe1fb26df092e2985937251a49feadc61eb0
Signed-off-by: Zheng Bao <zheng.bao@amd.com>
Signed-off-by: Zheng Bao <fishbaozi@gmail.com>
Reviewed-on: http://review.coreboot.org/11600
Tested-by: build bot (Jenkins)
Reviewed-by: Patrick Georgi <pgeorgi@google.com>

0 files changed, 0 insertions, 0 deletions