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.

boot/grub2/gen-grub-cfg.sh

@@ -8,6 +8,6 @@ kernel_filename=$1
 cat > "${TARGET_DIR}"/boot/grub.cfg <<EOF
 menuentry "Aimee OS ${vers} (Buildroot ${br2_vers}) on \$dev" --class gnu-linux --id "id-\$partuuid" "\$dev" "\$partuuid" {
     set root="\$2"
-    linux "/boot/${kernel_filename}" root=PARTUUID=\$3 ro
+    linux "/boot/${kernel_filename}" root=PARTUUID=\$3 ro \$rootflags
 }
 EOF