build: Cleanup nam configuration and enable by default

Removed unnecessary conditional for non-ota packages. Integrated checks
to make sure NAM_VARIANT did not override CM_BUILD and CM_BUILD did not
override NAM_VARIANT.
Added to all cm builds, since the script will be available and will not
interfere with devices that do not need it, but also to all htc
devices, which will usually need it.
Enabled the script by default, since it will not hurt devices that do
not require the configuration.