grub2: Support setting rootflags in config

The `AIMEEOS_DEFAULT_ROOTFLAGS` kconfig option can be used to set the
default value for the `rootflags` GRUB2 environment variable.  The value
of this variable will be passed along as command-line arguments to
whatever kernel is chosen at boot.

Since post-image scripts do not have access to kconfig option values, we
need to build the GRUB2 environment file in the
`GRUB2_INSTALL_IMAGES_CMDS` script fragment instead.

board/qemu/post-image.sh

@@ -3,16 +3,12 @@
 set -e
 
 BOARD_DIR=$(dirname "$0")
-GRUBENV="${BINARIES_DIR}"/efi-part/EFI/BOOT/grubenv
 
 genimage_tmp=$(mktemp -d)
 trap 'rm -rf "${genimage_tmp}"' EXIT
 
 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" \