This adds compile-time logic to disable certain frontends in mini-os:
 - pcifront is disabled by default, enabled for ioemu
 - blkfront, netfront, fbfront, kbdfront, consfront are enabled by default
 - xenbus is required for any frontend, and is enabled by default

If all frontends and xenbus are disabled, mini-os will run without
needing to communicate with xenstore, making it suitable to run the
xenstore daemon. The console frontend is not required for the initial
console, only consoles opened via openpt or ptmx.

Signed-off-by: Daniel De Graaf <dgdegra@tycho.nsa.gov>
Acked-by: Ian Campbell <ian.campbell@citrix.com>
Cc: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Committed-by: Ian Jackson <Ian.Jackson@eu.citrix.com>