cmake: Merge extra Zephyr modules from multiple sources

Extra Zephyr modules can be provided via the EXTRA_ZEPHYR_MODULES CMake
list variable. Previously, if this variable were set in multiple places
(e.g. local CMake variable, cache variable set via -D, environment
variable), Zephyr will pick one source based on zephyr_get's documented
hierarchy and ignore the others.

Add the MERGE keyword when EXTRA_ZEPHYR_MODULES is read so that the list
of Zephyr modules is merged from all sources if this variable is set in
multiple places.

An example where this can be useful is when integrating additional Zephyr
modules with a vendor's Zephyr-based application whose build system already
contains usage of EXTRA_ZEPHYR_MODULES.

Signed-off-by: Kesavan Yogeswaran <hikes@google.com>