geli taste: allow GELIBOOT tagged providers as well

Currently the installer will tag geliboot partitions with both BOOT and
GELIBOOT; the former allows the kernel to taste it at boot, while the latter
is what loaders keys off of.

However, it seems reasonable to assume that if a provider's been tagged with
GELIBOOT that the kernel should also take that as a hint to taste/attach at
boot. This would allow us to stop tagging GELIBOOT partitions with BOOT in
bsdinstall, but I'm not sure that there's a compelling reason to do so any
time soon.

Reviewed by: oshogbo
Differential Revision: https://reviews.freebsd.org/D23387