[build] Generate random build identifier

[build] Generate random build identifier

Randomly generate a 32-bit build identifier that can be used to
identify identical iPXE ROMs when multiple such ROMs are present in a
system (e.g. when a multi-function NIC exposes the same iPXE ROM image
via each function's expansion ROM BAR).

Signed-off-by: Michael Brown <mcb30@ipxe.org>