Unless --external-sources is given, only download distfiles that
need to be included in srcfs. The rest will be downloaded anyway
by the bootstrap system once it gets network access.
To accomplish this, instead of searching steps for sources files,
we now parse steps/manifest. As a side effect, source_manifest.py
now outputs source files in the order they appear in the manifest.
* Support specifying the size of the target disk image for qemu
* For bare metal, only pad the image to the next megabyte
* Use truncate() to extend images, instead of writing zeros (faster)
* Return None from get_disk() with nonexistent name
* Leave 1MiB on non-boot disks, or 1GiB on boot disks, unpartitioned
(for proper 4K alignment and to help preserve the srcfs or boot
partition creation)
* Fix qemu invocation when an external.img is not used
* Make -qr work with kernel bootstrap (will need kexec-fiwix fix)