Allow ROOT_MNJRO to reside on different device

Currently, the installer seems to be restricted so that BOOT_MNJRO and ROOT_MNJRO reside on the same device. This is unfortunate, as I installed an NVMe M.2 SSD into my PineBook Pro. As it seems, the PBP currently cannot boot from that device, but I guess it should be possible to boot from the internal eMMC storage (or MicroSD card) but have the root partition reside on the NVMe SSD. Would it be possible to implement that into the installer and also support LUKS for that separate root partition? I guess there're some people out there who would prefer not to rely on the eMMC for daily use.