nanobsd: Use makefs -R flag to honor rounded sizes

Switch from the min/max size flag (-s) to the round-up flag (-R) when
invoking makefs(8).

Because the partition sizes passed to nano_makefs have already been
rounded up by calculate_partitioning(), using -s can cause makefs to try
to perform sizing adjustments that usually result in failures.

Reviewed by:	imp
MFC after:	2 weeks
Differential Revision:	https://reviews.freebsd.org/D57225

tools/tools/nanobsd/defaults.sh

@@ -719,7 +719,7 @@ nano_makefs() {
 	dir=$5
 
 	makefs ${options} -F "${metalog}" -N "${NANO_WORLDDIR}/etc" \
-	    -s "${size}b" -T "${NANO_TIMESTAMP}" -t ffs "${image}" "${dir}"
+	    -R "${size}b" -T "${NANO_TIMESTAMP}" -t ffs "${image}" "${dir}"
 }
 
 # Convenient spot to work around any umount issues that your build environment