scripts/gen_image_generic.sh: replace deprecated backticks with $()

This replaces deprecated backticks by more versatile $(...) syntax.

Found with shellcheck.

Signed-off-by: Rosen Penev <rosenp@gmail.com>
[adjust commit title and message]
Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>

scripts/gen_image_generic.sh

@@ -20,7 +20,7 @@ sect=63
 cyl=$(( (KERNELSIZE + ROOTFSSIZE) * 1024 * 1024 / (head * sect * 512)))
 
 # create partition table
-set `ptgen -o "$OUTPUT" -h $head -s $sect -p ${KERNELSIZE}m -p ${ROOTFSSIZE}m ${ALIGN:+-l $ALIGN} ${SIGNATURE:+-S 0x$SIGNATURE}`
+set $(ptgen -o "$OUTPUT" -h $head -s $sect -p ${KERNELSIZE}m -p ${ROOTFSSIZE}m ${ALIGN:+-l $ALIGN} ${SIGNATURE:+-S 0x$SIGNATURE})
 
 KERNELOFFSET="$(($1 / 512))"
 KERNELSIZE="$2"