Build: Fix more bugs in the POSIX sh-ification of scripts

Kernel/build-gpt-image-grub.sh

@@ -73,7 +73,7 @@ mkdir -p mnt/boot
 echo "done"
 
 echo "installing grub using $grub..."
-$grub --boot-directory=mnt/boot --target=i386-pc --modules="ext2 part_msdos part_gpt ${dev}" 
+$grub --boot-directory=mnt/boot --target=i386-pc --modules="ext2 part_msdos part_gpt" "${dev}"
 
 if [ -d mnt/boot/grub2 ]; then
     cp grub_gpt.cfg mnt/boot/grub2/grub.cfg

Kernel/build-image-grub.sh

@@ -73,7 +73,7 @@ mkdir -p mnt/boot
 echo "done"
 
 echo "installing grub using $grub..."
-$grub --boot-directory=mnt/boot --target=i386-pc --modules="ext2 part_msdos ${dev}"
+$grub --boot-directory=mnt/boot --target=i386-pc --modules="ext2 part_msdos" "${dev}"
 
 if [ -d mnt/boot/grub2 ]; then
     cp grub.cfg mnt/boot/grub2/grub.cfg