runqemu: fix run from testimage with non-standard DEPLOY_DIR_IMAGE

testimage.bbclass uses runqemu to execute runtime tests on a qemu
target, this means that bitbake is already running and `bitbake -e`
can't be called to obtain bitbake variables.

runqemu tries to work around being unable to read values for
bitbake variables by inferring the MACHINE from the
DEPLOY_DIR_IMAGE setting, however if a user sets that variable in
a manner which doesn't follow the systems expectations (i.e. if
running `bitbake -c testimage` against a directory of pre-generated
images in a user-specified path) the inferring of the MACHINE name
from the DEPLOY_DIR_IMAGE location will fail.

It's possible that check_arg_machine() shouldn't cause runqemu to
fail and that runqemu should proceed with the user-supplied value
even if it can't be verified. This patch simply ensures that a
workflow where the user sets DEPLOY_DIR_IMAGE continues to work
without changing too much of the runqemu code.

[YOCTO #10238]

(From OE-Core rev: f94ac02f459e2ea0fc471463966997814a67e0ca)

Signed-off-by: Joshua Lock <joshua.g.lock@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

scripts/runqemu

@@ -277,6 +277,19 @@ class BaseConfig(object):
         elif self.get('MACHINE') == arg:
             return
         logger.info('Assuming MACHINE = %s' % arg)
+
+        # if we're running under testimage, or similarly as a child
+        # of an existing bitbake invocation, we can't invoke bitbake
+        # to validate the MACHINE setting and must assume it's correct...
+        # FIXME: testimage.bbclass exports these two variables into env,
+        # are there other scenarios in which we need to support being
+        # invoked by bitbake?
+        deploy = os.environ.get('DEPLOY_DIR_IMAGE', None)
+        bbchild = deploy and os.environ.get('OE_TMPDIR', None)
+        if bbchild:
+            self.set_machine_deploy_dir(arg, deploy)
+            return
+
         cmd = 'MACHINE=%s bitbake -e' % arg
         logger.info('Running %s...' % cmd)
         self.bitbake_e = subprocess.Popen(cmd, shell=True, stdout=subprocess.PIPE).stdout.read().decode('utf-8')