bld_utils: Fix warnings reported by pylint

 W: 47,0: Unused import copy
 W: 50,0: Unused import signal
 W: 53,0: Unused import urllib
 W: 75,36:DirRenamer.__exit__: Redefining name 'traceback' from outer scope (line 57)
 W: 75,23:DirRenamer.__exit__: Redefining built-in 'type'
 W: 92,14:stripVars: Redefining built-in 'object'
 W:113,4:urllib2_response_read: Redefining built-in 'file'
 C:143,16:download: More than one statement on a single line
 W:200,0:getEnvironment: Dangerous default value {} as argument
 W:317,0:runInstallCommand: Dangerous default value {} as argument
 W:331,0:runBuildCommand: Dangerous default value {} as argument
 W:340,0:getReturnValue: Dangerous default value {} as argument

Change-Id: I7aff0fec7c9a068a163e18bd3dbff6aca47e7de2
Reviewed-by: Iikka Eklund <iikka.eklund@digia.com>