board/freescale/common/imx/imx8-bootloader-prepare.sh: enable error checking

Run script with 'errexit' bash option to detect any errors from
subcommands.

It will fix situation where 'ddr_fw.bin' was missing but successfull
build created broken boot image. Post image script report this by:

cat: /home/user/buildroot/output/images/ddr_fw.bin: No such file or directory

and build finish with success.

Signed-off-by: Wojciech Nizinski <wojciech.nizinski@grinn-global.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>

board/freescale/common/imx/imx8-bootloader-prepare.sh

@@ -73,4 +73,5 @@ main ()
 	exit $?
 }
 
+set -e
 main $@