west build: warn about conditional flags in 'extra_configs'

The 'west build' command does not know about conditional flags (in the
format 'type:value:CONFIG_FOO=bar') in the 'extra_configs' argument of
Twister testcase.yaml files, and currently converts them to malformed
arguments that are silently ignored by cmake.

This change adds a check to 'west build' to clearly warn the user if the
'extra_configs' list contains conditional flags and provide a hint on
how to add them to the CMake command line.

Signed-off-by: Luca Burelli <l.burelli@arduino.cc>