MFC r339658:
loader: biosdisk interface should be able to cope with 4k sectors

The 4kn support in current bios specific biosdisk.c is broken, as the code
is only implementing the support for the 512B sector size.

This work is building the support for custom size sectors, we still do assume
the requested data to be multiple of 512B blocks and we only do address the
biosdisk.c interface here.

For reference, see also:
https://www.illumos.org/issues/8303
https://www.illumos.org/rb/r/547

As the GELI is moved above biosdisk "layer", the GELI should just work

Reviewed by: imp
Differential Revision: https://reviews.freebsd.org/D11174