[build] Reinstate the .pdsk padded-floppy image format
Some utilities that expect a floppy disk image (e.g. iLO?) may test
for a file of the correct size. Reinstate the .pdsk image format in
order to provide this if needed.
[build] Pad .rom, .dsk, and .hd images to 512-byte boundaries
QEMU will silently round down a disk or ROM image file to the nearest
512 bytes. Fix by always padding .rom, .dsk and .hd images to the
nearest 512-byte boundary.
Originally-fixed-by: Stefan Hajnoczi <stefanha@gmail.com>