rpmostreepayload: Rework remote add handling

We really want the remote to be in /etc/ostree/remotes.d, so
that it can be easily manipulated post-install.

To do that unfortunately is a complex dance.
rpmostreepayload: Handle grub2+EFI layout

We previously assumed if /boot/grub2 exists it's in use, but we
actually need to check for /boot/grub2/grub.cfg, as on EFI the config
file is in /boot/EFI.
bootloader: Bridge efi_dir configuration earlier for rpmostreepayload

rpmostreepayload wants a stub bootloader configuration to be written,
then it updates it.  There is a bit of magic in the middle of
bootloader.py to bridge the install class' efi_dir to the bootloader
configuration - we just need to set this up before calling
writeBootLoaderFinal().

Otherwise the config ends up in BOOT, which doesn't work.