Fix MFS builds when both MD_ROOT_SIZE and MFS_IMAGE are specified

MD_ROOT_SIZE and embed_mfs.sh were basically retired as part of
https://reviews.freebsd.org/D2903 .
However, when building a kernel with 'options MD_ROOT_SIZE' specified, this
results in a non-working MFS, as within sys/dev/md/md.c we fall within the
wrong # ifdef.

This patch implements the following:

• Allow kernels to be built without the MD_ROOT_SIZE option, which results in a kernel built as per D2903.
• Allow kernels to be built with the MD_ROOT_SIZE option, which results in a kernel built similarly to the pre-D2903 way, with the following differences:
  • The MFS is now put in a separate section within the kernel (oldmfs, so it differs from the mfs section introduced by D2903).
  • embed_mfs.sh is changed, so it looks up the oldmfs section within the kernel, gets its size and offset, sees if the MFS will fit within the allocated oldmfs section and only if all is well does a dd of the MFS image into the kernel.

Submitted by: Stanislav Galabov <sgalabov@gmail.com>
Reviewed by: brooks, imp
Differential Revision: https://reviews.freebsd.org/D5093