mconf-idf: Use same 'simple expand' logic, same as kconfig-frontends

mconf-idf: Use same 'simple expand' logic, same as kconfig-frontends

Previously, wordexp() was used. However for providing Windows builds
of mconf-idf we can't use wordexp() so we use this simplified
environment variable expansion code instead.

The reasoning here is to make the behaviour consistent across Windows
(CMake vs GNU Make), Linux & macOS mconf.