firmware: Introduce CONFIG_ROMBIOS and CONFIG_SEABIOS options

firmware: Introduce CONFIG_ROMBIOS and CONFIG_SEABIOS options

This patch introduces configuration options allowing to built either a
rombios only or a seabios only hvmloader.

Building option ROMs like vgabios or etherboot is only enabled for a
rombios hvmloader, since SeaBIOS takes care or extracting option ROMs
itself from the PCI devices (these option ROMs are provided by the
device model and do not need to be built in hvmloader).

The Makefile in tools/firmware/ now only checks for bcc if rombios is
enabled.

These two configuration options are left on by default to remain
compatible.

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