tools/include: Rework Makefile

tools/include: Rework Makefile

Rework "xen-xsm" rules to not have to change directory to run
mkflask.sh, and store mkflask.sh path in a var, and use a full path
for FLASK_H_DEPEND, and output directory is made relative.

Rename "all-y" target to a more descriptive "xen/lib/x86/all".

Removed the "dist" target which was the only one existing in tools/.

Signed-off-by: Anthony PERARD <anthony.perard@citrix.com>
Acked-by: Andrew Cooper <andrew.cooper3@citrix.com>
Release-acked-by: Henry Wang <Henry.Wang@arm.com>