postmarketos-mkinitfs-hook-debug-shell: add debugfs mount

postmarketos-mkinitfs-hook-debug-shell

Add debugfs mount - very useful in debugging issues.

I'm testing this for a few days and it is so convenient that I don't have to type this again every new reboot to debug-shell:

$ mount -t debugfs none /sys/kernel/debug

😇

Also checking regulators/gpios is not faster:

$ cat /d/gpio
$ cat /d/regulator/regulator_summary

instead of

$ cat /sys/kernel/debug/gpio
$ cat /sys/kernel/debug/regulator/regulator_summary