build: build-root/Makefile rm of install tree

build: build-root/Makefile rm of install tree

Type: fix

The scenario is touch of a dpdk source file in places like
<TOP>/build-root/build-vpp-native/external/dpdk-19.08/.  The subsequent
build will trigger an rm -rf $(PACKAGE_INSTALL_DIR) by
build-root/Makefile at around line 709.  Because the package in
question, "external", consists of various "sub" packages, all of which
are controlled with files like .quicly.install.ok located in
build-root/build-vpp[_debug]-native/external, we end up not handling the
.*.install.ok files to reflect the removal of that PACKAGE_INSTALL_DIR.
This fix will correct that, at least for the case of touching/changing
dpdk files. A better long term solution would be to change the "sub"
packages to be full fledged build system packages, as Dave Barach points
out.

Signed-off-by: Burt Silverman <burtms@gmail.com>
Change-Id: Ia94ccaa2795cbf354b5921c3001a7141313b66be