change host ipxe file names and avoid stale files

change host ipxe file names and avoid stale files

If a server is build multiple times but with different names or
different nics selected, the ipxe.efi will contain out of date
configuration files that may be matched before the desired settings.
Changed to use hostname_mac.ipxe and deletes any old hostname_* or
*_mac files before creating the new file with the new settings.

Signed-off-by: davidplunkett <dp7642@att.com>
Change-Id: I5b4f35b5234ce90ad9f13c785c0c9eaf81bfa13a