Support for configured platform in lb-dev projects

With the removal of the default platform in user environments, lb-dev generated projects must be able to work without BINARY_TAG or CMTCONFIG environment variables.

The changes proposed are:

• add support for -c best (or undefined platform) to lb-dev (cca56f23)
• add platform field in the special build.conf file, and use it if the platform environment variables are not set (01eba2d3)
• fix the special script run to use the platform from build.conf (aa14b13f)

Note that aa14b13f also includes fixes for CMT projects (it looks like they never worked since the switch to LbDevTools) that I needed to validate this MR.

Closes LBCORE-1861