tools: fix dependency for ipxe and rombios

tools: fix dependency for ipxe and rombios

It appears that the test in 01d631028 for ipxe's dependency on rombios
is not good enough. Configuring with --disable-rombios doesn't disable
ipxe.

Fix it by testing the dependency after AC_ARG_ENABLE and AC_ARG_WITH
have taken effect.

At the same time, regularise options for ipxe:

  --enable-ipxe         enable building in-tree ipxe
  --disable-ipxe        disable building in-tree ipxe
  --with-system-ipxe    specify a path to be baked into code, disable
                        building in-tree ipxe, this trumps --{en,dis}able-ipxe
  --without-system-ipxe error

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