bsdinstall: increase EFI partition size to 200MB

A larger EFI file system size will facilitate multi-boot configurations
and the installation other EFI applications like firmware update tools.
200MB matches OS X.

Note that this changes only the partition size, not the file system that
bsdinstall places there. We need to do both, but as the partition size
is difficult to adjust later make this change for now so that at least
systems installed with FreeBSD 11.0 have a partition layout with room
to grow.

Reviewed by: allanjude, imp
Approved by: re (gjb)
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D6935