main/postmarketos-mkinitfs: mount /sysroot/boot (MR 1398)

Mount the boot partition at /sysroot/boot and keep it mounted, when
running the switch_root command. This way, OpenRC doesn't need to mount
it and possibly use the wrong partition. The OpenRC service does not use
the same logic to find the boot partition, in particular it does not
support the pmos_boot kernel parameter.

While at it, print the mountpoint and read-only/read-write arguments in
the mounting log message for both root and boot.

Fixes: #664