build: more adjustments to top-level Makefile dependencies

build: more adjustments to top-level Makefile dependencies

In the original code, top-level dist target unconditionally invokes
dist target for tools/include, which is wrong when tools component is
not enabled.

Make dist-tools depend on dist-tools-public-headers, which depends on
build-tools-public-headers.

Discovered by Travis-CI.

Signed-off-by: Wei Liu <wei.liu2@citrix.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>
elease-acked-by: Julien Grall <julien.grall@arm.com>