oeqa/selftest/esdk: remove postconfig argument

oeqa/selftest/esdk: remove postconfig argument

Discovered when looking for users of get_bb_var()'s postconfig argument,
this wrapper around runCmd() has a postconfig argument that has odd
behaviour: it _appends_ the new configuration to local.conf instead of
having them used for this specific run (unlike the other functions in
commands.py)

None of the eSDK tests use this functionality, so remove it. Future test
cases that need to write further configuration should do so directly.

Signed-off-by: Ross Burton <ross.burton@arm.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>