11277 Loader cannot boot from root pool with encryption enabled

From FreeBSD: https://svnweb.freebsd.org/base?view=revision&revision=349217



Tell loader to ignore newer features enabled on the root pool.




There are many new features in ZoF. Most, if not all, do not effect read only usage.

Encryption in particular is enabled at the pool level but used at the dataset level.

The loader obviously will not be able to boot if the boot dataset is encrypted, but

should not care if some other dataset in the root pool is encrypted.

Tested on OmniOS - used to fix a non-booting VM after creating a test encrypted dataset - rpool/enc