main/postmarketos-initramfs: add cmdline to expose specific USB storage (MR 5912)

In addition to the pmos.debug-shell cmdline argument, add
pmos.usb-storage=<block-device> to set up the USB mass storage gadget by
default when going into debug shell. This allows building a special boot
image/option that directly exposes the storage for flashing, without having
to type in extra debug-shell commands.