This solves the issue that target is changed to the default one after
idf.py fullclean.
Also allow setting the default target using sdkconfig.defaults, e.g.
CONFIG_IDF_TARGET="esp32s2"
Closes IDF-1040
Python 2 expect the environ variables are all of type 'str', but
sometimes wrong 'unicode' type is given.
Here we force all variables that are not str to become str.