tools: consolidate Makefile runes to building+installing qemu-trad

tools: consolidate Makefile runes to building+installing qemu-trad

Currently both subdir-all-qemu-traditional-dir and subdir-install-...
recurse into qemu-traditional-dir and run xen-setup, but with subtly
different options (--extra-cflags was only passed for install).

Consolidate these by having the install target depend on the all
(build) target and only calling make install.

There is a subtlty which is that qemu-trad's build is not completely
isolated from xen.git and requires some environment variables etc. To
handle this refactor the setup code into a make macro and $(call) it
in both cases.

Note that this doesn't actually omit a potential double call to
configure, since all is still called during install, this is the same
for qemu-xen. Note that it doesn't result in a double build in either
case, since apparently the qemu configure is idempotent.

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>