Makefile: Fix clobber of LIB_INCLUDE_DIR during second pass

The commit:
commit f25ac092
 "Makefile: Restructure for multilibs"

breaks external HAL library linking, by overriding, in the
the second Makefile pass, the LIB_INCLUDE_DIR variable
set in the first Makefile pass from ext/hal Makefiles.

This breaks use of the CONFIG_CC3200SDK_LIBRARY=y option in
the cc3200 SDK, and also likely the QMSI ext/hal/qmsi/Makefile,
when CONFIG_QMSI_LIBRARY=y is set.

This commit just changes the '=' to "+=".

Jira: ZEP-1163

Change-Id: I1aaaaee57378dcd01ab6f2ed4799281b2337eea2
Signed-off-by: Gil Pitney <gil.pitney@linaro.org>