bcm27xx: improve image generation script

- Exit immediately on a non-zero status.
- Remove empty lines.
- Remove unused variables.
- Add missing ptgen variables for readability.
- Refactor SYNTAX if block.

(cherry picked from commit 96b03ff2476edb4d08fb4b91fdf80be7aa95ddd8)
Signed-off-by: Álvaro Fernández Rojas <noltari@gmail.com>

target/linux/bcm27xx/image/gen_rpi_sdcard_img.sh

@@ -1,10 +1,11 @@
 #!/bin/sh
 
-set -x
-[ $# -eq 5 ] || {
+set -e -x
+
+if [ $# -ne 5 ]; then
     echo "SYNTAX: $0 <file> <bootfs image> <rootfs image> <bootfs size> <rootfs size>"
     exit 1
-}
+fi
 
 OUTPUT="$1"
 BOOTFS="$2"
@@ -12,18 +13,16 @@ ROOTFS="$3"
 BOOTFSSIZE="$4"
 ROOTFSSIZE="$5"
 
+align=4096
 head=4
+kernel_type=c
+rootfs_type=83
 sect=63
 
-set $(ptgen -o $OUTPUT -h $head -s $sect -l 4096 -t c -p ${BOOTFSSIZE}M -t 83 -p ${ROOTFSSIZE}M)
+set $(ptgen -o $OUTPUT -h $head -s $sect -l $align -t $kernel_type -p ${BOOTFSSIZE}M -t $rootfs_type -p ${ROOTFSSIZE}M)
 
 BOOTOFFSET="$(($1 / 512))"
-BOOTSIZE="$(($2 / 512))"
 ROOTFSOFFSET="$(($3 / 512))"
-ROOTFSSIZE="$(($4 / 512))"
 
 dd bs=512 if="$BOOTFS" of="$OUTPUT" seek="$BOOTOFFSET" conv=notrunc
 dd bs=512 if="$ROOTFS" of="$OUTPUT" seek="$ROOTFSOFFSET" conv=notrunc
-
-
-