scripts: kconfig: Always generate ZEPHYR_*_MODULE

Define the ZEPHYR_{MODULE_NAM}_MODULE option for all available modules.
Right now only modules defined as 'kconfig-ext' has this symbol defined
which may cause problem if the build wants to check if a module not
using this options is available.

This allow us to do something like:

comment "mbedTLS module not available."
	depends on !ZEPHYR_MBEDTLS_MODULE

or make a feature depends on the module be present.

Signed-off-by: Flavio Ceolin <flavio.ceolin@intel.com>