nanobsd: Add a nano_makefs function

Add a NANO_MAKEFS global variable with options equivalent to NANO_NEWFS
to be used with a nano_makefs function for creating images.

Also add a function that adjusts the code size calculation, so makefs -s
won't error about the minimum rounded size. Ideally this shim should be
removed, therefore the suspicious _xxx prefix.

Initially not wired.

Reviewed by: imp
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D48789