Buildsystem: default bootloader is not my custom one

You can set BANAN_BOOTLOADER=GRUB to use grub instead. Image creation
does not convert disk image now automatically between bootloaders and
calling ./bos image-full is now required.

script/config.sh

@@ -25,3 +25,7 @@ export BANAN_DISK_IMAGE_PATH=$BANAN_BUILD_DIR/banan-os.img
 if [[ -z $BANAN_UEFI_BOOT ]]; then
 	export BANAN_UEFI_BOOT=0
 fi
+
+if [[ -z $BANAN_BOOTLOADER ]]; then
+	export BANAN_BOOTLOADER="BANAN"
+fi