recovery: indicate that this is a recovery boot

[osresearch]

the boot mode should be displayed if the disk key is required for any reason

[osresearch]

added a small message in https://github.com/osresearch/safeboot/commit/8f0a5327e7ed0b0ae9fb94b33749203ebf16a0f6

Ideally the entire background would change, similar to the warnings provided by Android: https://source.android.com/security/verifiedboot/boot-flow

The vendor logo and uefi drawing is part of the "flicker free" boot: https://hansdegoede.livejournal.com/20632.html

Files for gui stuff there are in /usr/share/plymouth/themes/

[osresearch]

Recovery shell now has ASCII art to indicate that it is not the real boot.