raspberrypi: post-image.sh fix gpu_mem option

The gpu_mem option is not using the proper option argument which causes sed to
fail.

	+ case "${arg}" in
	+ gpu_mem=ome/gportay/output-rpi3-qtwe/images
	+ sed -e '/^ome/gportay/output-rpi3-qtwe/images=/s,=.*,=ome/gportay/output-rpi3-qtwe/images,' -i /home/gportay/output-rpi3-qtwe/images/rpi-firmware/config.txt
	sed: -e expression #1, char 8: extra characters after command
	+ case "${arg}" in
	+ gpu_mem=ome/gportay/output-rpi3-qtwe/images
	+ sed -e '/^ome/gportay/output-rpi3-qtwe/images=/s,=.*,=ome/gportay/output-rpi3-qtwe/images,' -i /home/gportay/output-rpi3-qtwe/images/rpi-firmware/config.txt
	sed: -e expression #1, char 8: extra characters after command

The issue comes from the use of $1 instead of $arg to extract the gpu_mem
value. $1 is the $(BINARIES_DIR) which leads to a sed expression error.

Also, it adds the error flag to the shell script to prevent from such situation
and terminate the build in error.

Signed-off-by: Gaël PORTAY <gael.portay@savoirfairelinux.com>
Acked-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>