Enable debug logging for GRUB rescue shell options

If the system fails to boot, some kernel messages may not be shown before the
system fully boots. Enable the debug option for rescue shell options to have an
easy way to see those without modifying GRUB options. This will increase log
verbosity and turn on debug logging for Systemd as well [1].

[1] https://www.freedesktop.org/software/systemd/man/latest/systemd.html#debug

buildroot-external/board/pc/grub.cfg

@@ -77,9 +77,9 @@ menuentry "Slot B (OK=$B_OK TRY=$B_TRY)" {
 }
 
 menuentry "Slot A (rescue shell)" {
-    linux (${boothd},gpt2)/bzImage root=PARTUUID=8d3d53e3-6d49-4c38-8349-aff6859e82fd $default_cmdline $cmdline rauc.slot=A systemd.unit=recovery.target
+    linux (${boothd},gpt2)/bzImage root=PARTUUID=8d3d53e3-6d49-4c38-8349-aff6859e82fd $default_cmdline $cmdline rauc.slot=A systemd.unit=recovery.target debug
 }
 
 menuentry "Slot B (rescue shell)" {
-    linux (${boothd},gpt4)/bzImage root=PARTUUID=a3ec664e-32ce-4665-95ea-7ae90ce9aa20 $default_cmdline $cmdline rauc.slot=B systemd.unit=recovery.target
+    linux (${boothd},gpt4)/bzImage root=PARTUUID=a3ec664e-32ce-4665-95ea-7ae90ce9aa20 $default_cmdline $cmdline rauc.slot=B systemd.unit=recovery.target debug
 }