chore: update Grub boot scripts

boot/grub/first.grub.cfg

@@ -0,0 +1,26 @@
+set rugpi_bootpart=2
+set rugpi_boot_spare=false
+
+set RUGPI_DEFAULT_ENV=(${root})/rugpi/bootpart.default.grubenv
+set RUGPI_SPARE_ENV=(${root})/rugpi/boot_spare.grubenv
+
+load_env -f "${RUGPI_DEFAULT_ENV}"
+load_env -f "${RUGPI_SPARE_ENV}"
+
+if [ "${rugpi_boot_spare}" = "true" ]; then
+    if [ "${rugpi_bootpart}" = "2" ]; then
+        set rugpi_bootpart=3
+    elif [ "${rugpi_bootpart}" = "3" ]; then
+        set rugpi_bootpart=2
+    fi
+
+    set rugpi_boot_spare=false
+    save_env -f "${RUGPI_SPARE_ENV}" rugpi_boot_spare
+    set rugpi_boot_spare=true
+fi
+
+# TODO: This should not be hardcoded to `hd0`, either use `search` or extract the correct
+# root drive from the existing value of `root` in some way.
+set root=(hd0,${rugpi_bootpart})
+
+source (${root})/second.grub.cfg

boot/grub/second.grub.cfg

@@ -0,0 +1,3 @@
+linux /vmlinuz ro init=/usr/bin/rugpi-ctrl
+initrd /initrd.img
+boot