cmake: fix Zephyr module ext order

MODULE_EXT_ROOT allows Zephyr modules to provide glue code for Zephyr
modules using `cmake-ext: True` and/or `kconfig-ext:True`.

A module ext root provides a `modules.cmake` file which defines
variables like: `set(ZEPHYR_FOO_CMAKE_DIR  <glue-code-path>/foo)`

It is intended that a downstream module can replace Zephyr's default
glue code, which again can be replaced further downstream.

Setting values in first modules.cmake (Zephyr's) are replaced by later
processed modules.cmake (downstream) when the setting name is identical.

Therefore the module ext root list should not be reversed, and Zephyr
itself should be placed as first entry in the list.

Signed-off-by: Torsten Rasmussen <Torsten.Rasmussen@nordicsemi.no>