pmb/install: Add option to create single combined boot/root partition

With recent rework in postmarketos-initramfs, we no longer need to mount the /boot partition in the initramfs (assuming initramfs-extra is not used). On devices that boot without accessing the boot file system (e.g. Android boot images, fastboot, ...), that makes it possible to install postmarketOS on a single (potentially encrypted) partition that contains both root (/) and /boot files.

This avoids the extra complexity of the subpartition setup we usually use on such devices, and also avoids having to flash two partitions (when using --split to avoid the subpartitions).

Add a --single-partition option to pmbootstrap install that allows installing postmarketOS in this mode. For now this is just an option that must be selected explicitly, in the future we could choose to make this the default for Android-based devices with a large enough boot partition.