oeqa/utils/commands.py: add support for postconfig option

oeqa/utils/commands.py: add support for postconfig option

Adding support for postconfig option to the bitbake() and related methods.

This enables us to use 'bitbake -R postconfig_file <command>'.

Usage: bitbake(cmd, postconfig="some confguration")

'postconfig_file' would contain what we add in 'postconfig'

Other methods affected: get_bb_env(), get_bb_var()

Signed-off-by: Corneliu Stoicescu <corneliux.stoicescu@intel.com>
Signed-off-by: Saul Wold <sgw@linux.intel.com>