Log exported environnment variables with `-ldebug`

[ccollins476ad]

This PR adds some additional information to the debug log.

Here is an example from a newt build -ldebug <...> run:

2020/04/22 10:39:03.864 [DEBUG] exporting environment variables:
2020/04/22 10:39:03.865 [DEBUG]     BIN_ROOT=/Users/ccollins/proj/myproj/bin
2020/04/22 10:39:03.865 [DEBUG]     BSP_PATH=/Users/ccollins/proj/myproj/repos/apache-mynewt-core/hw/bsp/native
2020/04/22 10:39:03.865 [DEBUG]     CORE_PATH=/Users/ccollins/proj/myproj/repos/apache-mynewt-core
2020/04/22 10:39:03.866 [DEBUG]     MYNEWT_NEWT_PATH=/Users/ccollins/go/bin/newt
2020/04/22 10:39:03.866 [DEBUG]     MYNEWT_PROJECT_ROOT=/Users/ccollins/proj/myproj

[ccollins476ad]

Thanks @vrahane.