stubdom: correct handling DESTDIR during build

stubdom: correct handling DESTDIR during build

The stubdom/Makefile expects components to install themselves to
stubdom/$(CROSS_ROOT) and passes the appropriate configure and make
variables to make this happen.

However if the top-level build uses DESTDIR then this not only effects
the installation targets in stubdom/Makefile but is also propagated to
the individual components' build causing them to install into an
unexpected location and breaking the build.

In other words "make -C stubdom DESTDIR=/tmp/a-dest-dir" fails due to missing
headers because they had been installed into /tmp/a-dest-dir instead of
stubdom/$(CROSS_ROOT).

The existing attempt to clear DESTDIR for the newlib install phase was
insufficient.

Define a CROSS_MAKE variable which should be used to recurse into the
cross compiled components and which clears DESTDIR.

Also move the definition of the genpath target to after the first
"all" target so that the default target is all.

I have confirmed that with this fix only the Xen build system outputs end up in
/tmp/a-dest-dir.

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