Always issue the pxe request

All the code are now only issueing one single dhcp request at startup of the
loader meaning we can always request a the PXE informations from the
dhcp server.

Previous code lost that information, meaning no option 55 anymore (meaning not
working with the kea dhcp server) and no request for rootpath etc, no user class

Remove the flags from the bootp function which is not needed anymore

Reviewed by: tsoome
Differential Revision: https://reviews.freebsd.org/D10952