scripts/gen_image_generic.sh: fix more shellcheck warnings

scripts/gen_image_generic.sh: fix more shellcheck warnings

Fixes following shellcheck warnings:

 In scripts/gen_image_generic.sh line 20:
 cyl=$(( (KERNELSIZE + ROOTFSSIZE) * 1024 * 1024 / (head * sect * 512)))
 ^-^ SC2034: cyl appears unused. Verify use (or export if used externally).
 --
 In scripts/gen_image_generic.sh line 34:
     [ -n "$PADDING" ] && dd if=/dev/zero of="$OUTPUT" bs=512 seek="$(($ROOTFSOFFSET + $ROOTFSSIZE))" conv=notrunc count="$sect"
                                                                       ^-----------^ SC2004: $/${} is unnecessary on arithmetic variables.
 --
 In scripts/gen_image_generic.sh line 35:
     mkfs.fat -n kernel -C "$OUTPUT.kernel" -S 512 "$(($KERNELSIZE / 1024))"
                                                       ^---------^ SC2004: $/${} is unnecessary on arithmetic variables.

Signed-off-by: Petr Štetiar <ynezz@true.cz>