ext: libmetal: Change build integration so its not recursive

With recent changes to libmetal we can now include and build it directly
as a zephyr library rather than doing a recursive make.  We remove
ext/hal/libmetal.cmake as part of this change and introduce a Kconfig
option for libmetal.

This is a partial fix for issue #7673.

Signed-off-by: Kumar Gala <kumar.gala@linaro.org>