Add program main entry in apply_config.py

So make it easier to debug or run apply_config.py directly(no need to
install it).

Change-Id: Id0dad9cb02b81fd22d9868e686867af074aed966
Put _ ahead of unused variable subdirs

The variable is never used in loop, without `_' before the variable
name, pydev will mark a warning.

Change-Id: I3d63b0729c4ae9ec00ca97893c2ded65595235d1
Add blank char in fallback-metadata usage

Or the usage will be displayed as `...OS_CONFIG_FILESis...'. It should
be `...OS_CONFIG_FILES is...'.

Change-Id: I2e5dc1cb565da327efcbba1e7b07ed442ff583fd
Merge "Put _ ahead of unused variable subdirs"
Merge "Add blank char in fallback-metadata usage"
Merge "Add program main entry in apply_config.py"