directfb.inc: force bfd linker for armv7a

directfb.inc: force bfd linker for armv7a

Workaround for linker errors seen with armv7a + gold:

 | ../arm-rdk-linux-gnueabi-libtool  --tag=CC   --mode=link arm-rdk-linux-gnueabi-gcc  -march=armv7-a -mfloat-abi=hard -mtune=cortex-a15 --sysroot=.../build/tmp/sysroots/eos -I.../build/tmp/sysroots/eos/usr/include/freetype2 -I.../build/tmp/sysroots/eos/usr/include/libpng16 -Wall -Wstrict-prototypes -Wmissing-prototypes -Wno-strict-aliasing -Werror-implicit-function-declaration -O3 -g2 -ffast-math -pipe -O2 -pipe -g -feliminate-unused-debug-types -D_GNU_SOURCE  -std=gnu99 -Werror-implicit-function-declaration  -Wl,-O1 -Wl,--hash-style=gnu -Wl,--as-needed -o directfb-csource directfb-csource.o -lpng16 -ldl -lrt -lpthread
 | arm-rdk-linux-gnueabi-libtool: link: arm-rdk-linux-gnueabi-gcc -march=armv7-a -mfloat-abi=hard -mtune=cortex-a15 --sysroot=.../build/tmp/sysroots/eos -I.../build/tmp/sysroots/eos/usr/include/freetype2 -I.../build/tmp/sysroots/eos/usr/include/libpng16 -Wall -Wstrict-prototypes -Wmissing-prototypes -Wno-strict-aliasing -Werror-implicit-function-declaration -O3 -g2 -ffast-math -pipe -O2 -pipe -g -feliminate-unused-debug-types -D_GNU_SOURCE -std=gnu99 -Werror-implicit-function-declaration -Wl,-O1 -Wl,--hash-style=gnu -Wl,--as-needed -o directfb-csource directfb-csource.o  .../build/tmp/sysroots/eos/usr/lib/libpng16.so -lz -lm -ldl -lrt -lpthread
 | .../build/tmp/sysroots/x86_64-linux/usr/bin/arm-rdk-linux-gnueabi/../../libexec/arm-rdk-linux-gnueabi/gcc/arm-rdk-linux-gnueabi/5.2.0/ld: error: directfb-csource.o: requires unsupported dynamic reloc R_ARM_MOVW_ABS_NC; recompile with -fPIC
 | collect2: error: ld returned 1 exit status

Signed-off-by: Andre McCurdy <armccurdy@gmail.com>
Signed-off-by: Ross Burton <ross.burton@intel.com>