firmware: Use mkhex from hvmloader directory for etherboot ROMs

firmware: Use mkhex from hvmloader directory for etherboot ROMs

To remain consistent with how other ROMs are built into hvmloader,
call mkhex on etherboot ROMs from the hvmloader directory, instead of
the etherboot directory. In other words, eb-roms.h is not used any
more.

Introduce ETHERBOOT_NICS config option to choose which ROMs should be
built (kept rtl8139 and 8086100e per default as before).

Signed-off-by: Julian Pidancet <julian.pidancet@gmail.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>