oeqa/selftest/devtool: fix test_devtool_modify_overrides test

oeqa/selftest/devtool: fix test_devtool_modify_overrides test

This test fails for machines qemuarm and qemux86 because when doing
devtool modify, the default devtool branch contains the patch that match
the current configuration, so for both qemuarm and qemux86  machines the
corresponding override patch is applied and we get the following error
(for qemuarm machine):

  AssertionError: 'This is a test for qemuarm\n' != 'This is a test for something\n'
  - This is a test for qemuarm
  ?                    ^ ^^^^^
  + This is a test for something
  ?                    ^^^ ^^^^^

Fix the test by looking at the correct value depending on the current
machine configuration

Signed-off-by: Julien Stephan <jstephan@baylibre.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>