nanobsd: Use gpart and create code image before full disk image

The attached patch brings two main changes to the nanobsd script:
1- gpart is used instead of fdisk;
2- the code image is created first, and then used to ``assemble'' the

full disk image.

The patch was first proposed on the freebsd-embedded list:
http://lists.freebsd.org/pipermail/freebsd-embedded/2012-June/001580.html
and is currently under discussion:
http://lists.freebsd.org/pipermail/freebsd-embedded/2014-January/002216.html

Another effect is that the -f option ("suppress code slice extraction")
now imples the -i option ("suppress disk image build").

imp@ applied Patch by hand to new legacy.sh, plus tweaked for NANO_LOG vs
NANO_OBJ confusion in original.

PR: 186030
Reviewed by: imp@
Differential Revision: https://reviews.freebsd.org/D31102