What's a good way to go about this?

Can you use the same shell variables that get_dtb_path uses in cros-unibuild.eclass?

os.environ['SYSROOT'] ?

From what I can tell, because the filepath right now is passed in without a flag (no --filepath=...) it can't have a default value because of the sub-commands :( This means changing the CLI signature to use a --filepath flag over the <filepath> property. It complains that it can't tell if the filepath you just give it is supposed to be a command name or not. Recommendations?

Yes I think you should change that to a flag, like -c or --config and that way you can specify nothing (for the default), or:

-c /path/to/file
-c -                 (for stdin)

The following revision refers to this bug:
  https://chromium.googlesource.com/chromiumos/platform2/+/a561c4b3c8c0255cd9edc68253dca4a797616b5d

commit a561c4b3c8c0255cd9edc68253dca4a797616b5d
Author: Alec Thilenius <athilenius@chromium.org>
Date: Sat Oct 21 18:46:53 2017

chromeos-config: Change <filepath> to -c flag in cros_config_host_py

Change the required <filepath> property of cros_config_host_py to be an
optional --config flag. If the flag is not specified, the system configuration
will be used. Note that SYSROOT must be set for that to work.

BUG= chromium:774495 
TEST=FEATURES=test sudo -E emerge chromeos-base/chromeos-config-tools
CQ-DEPEND=CL:730765

Change-Id: I900398c00c0b91adf631a531002816dc98c2487d
Reviewed-on: https://chromium-review.googlesource.com/726301
Commit-Ready: Simon Glass <sjg@chromium.org>
Tested-by: Simon Glass <sjg@chromium.org>
Tested-by: Alec Thilenius <athilenius@chromium.org>
Reviewed-by: Simon Glass <sjg@chromium.org>

[modify] https://crrev.com/a561c4b3c8c0255cd9edc68253dca4a797616b5d/chromeos-config/cros_config_host_py/cros_config_host_unittest.py
[modify] https://crrev.com/a561c4b3c8c0255cd9edc68253dca4a797616b5d/chromeos-config/cros_config_host_py/cros_config_host.py

The following revision refers to this bug:
  https://chromium.googlesource.com/chromiumos/overlays/chromiumos-overlay/+/1ab6809096424ee88108ef7532fff53870cbb16d

commit 1ab6809096424ee88108ef7532fff53870cbb16d
Author: Alec Thilenius <athilenius@chromium.org>
Date: Sat Oct 21 18:46:52 2017

chromeos-config-tools: Update cros_config_host_py -c flag usage

In all eclass uses of cros_config_host_py, the -c flag was added where
the file path is passed in from stdin, and the entire call to
get_dtb_path was removed in other calls. cros_config_host_py now
supports useing the default config if no file is specified.

BUG= chromium:774495 
TEST=FEATURES=test sudo -E emerge  chromeos-config-tools
CQ-DEPEND=CL:726301

Change-Id: I6458cd4ac5eff46417e528bc34e637e6e1dce339
Reviewed-on: https://chromium-review.googlesource.com/730765
Commit-Ready: Simon Glass <sjg@chromium.org>
Tested-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Simon Glass <sjg@chromium.org>

[modify] https://crrev.com/1ab6809096424ee88108ef7532fff53870cbb16d/eclass/cros-unibuild.eclass
[modify] https://crrev.com/1ab6809096424ee88108ef7532fff53870cbb16d/eclass/cros-firmware.eclass