automation: fix builds with clang

automation: fix builds with clang

a75703b2f0f585a2fc6a7bcdb7d16a61bcf5e6b0 unconditionally enabled
building of rombios and stubdoms but unfortunately these two pieces do
not build successfully with clang. rombios unconditionally depends on
the build of iPXE and upstream iPXE does not support building with
clang. Similiarly the build of the stubdoms depends on the build of
newlib which does not support being built with clang.

Signed-off-by: Doug Goldstein <cardoe@cardoe.com>
Acked-by: Wei Liu <wei.liu2@citrix.com>