Revision 297691

Create the GELIBOOT GEOM_ELI flag

This flag indicates that the user wishes to use the GELIBOOT feature to boot from a fully encrypted root file system.
Currently, GELIBOOT does not support key files, and in the future when it does, they will be loaded differently.
Due to the design of GELI, and the desire for secrecy, the GELI metadata does not know if key files are used or not, it just adds the key material (if any) to the HMAC before the optional passphrase, so there is no way to tell if a GELI partition requires key files or not.

Since the GELIBOOT code in boot2 and the loader does not support keys, they will now only attempt to attach if this flag is set. This will stop GELIBOOT from prompting for passwords to GELIs that it cannot decrypt, disrupting the boot process

PR:		208251
Reviewed by:	ed, oshogbo, wblock
Sponsored by:	ScaleEngine Inc.
Differential Revision:	https://reviews.freebsd.org/D5867