tools: provide --with-system-ipxe

This option lets user specify which binary is to be used as ipxe. If
it is specified, the in-tree ipxe will not be built. This option is in
line with other --with-system-* options we provide.

Signed-off-by: Wei Liu <wei.liu2@citrix.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>

config/Tools.mk.in

@@ -50,6 +50,7 @@ FLASK_POLICY        := @xsmpolicy@
 CONFIG_OVMF         := @ovmf@
 CONFIG_ROMBIOS      := @rombios@
 CONFIG_SEABIOS      := @seabios@
+CONFIG_IPXE         := @ipxe@
 CONFIG_QEMU_TRAD    := @qemu_traditional@
 CONFIG_QEMU_XEN     := @qemu_xen@
 CONFIG_BLKTAP2      := @blktap2@