board/rpi3: post-image: Create grubenv BEFORE image

We obviously have to create the `grubenv` file before running
`genimage`, or it will not be included in the image.

board/raspberrypi3/post-image.sh

@@ -14,6 +14,9 @@ cp -alf "${BINARIES_DIR}"/rpi-firmware/* "${BINARIES_DIR}"/efi-part/
 
 mkdir -p  "${genimage_tmp}"/rootfs "${genimage_tmp}"/tmp
 
+grub-editenv "${GRUBENV}" set default=0
+grub-editenv "${GRUBENV}" set timeout=3
+
 cd "${BINARIES_DIR}"
 genimage \
 	--rootpath "${genimage_tmp}/rootfs" \
@@ -21,6 +24,3 @@ genimage \
 	--inputpath "${BINARIES_DIR}" \
 	--outputpath "${BINARIES_DIR}" \
 	--config "${BOARD_DIR}"/genimage.cfg
-
-grub-editenv "${GRUBENV}" set default=0
-grub-editenv "${GRUBENV}" set timeout=3