what is the actual problem?

https://chromium-review.googlesource.com/350095 was meant to exclude cr50 from buildall when building on the bot. 

cros-ec has public contributors that aren't bots :)

make[2]: *** No rule to make target 'private-cr51/build.mk'.  Stop.
Makefile.rules:115: recipe for target 'proj-cr50' failed
make[1]: *** [proj-cr50] Error 2
make[1]: *** Waiting for unfinished jobs....

It's related to this line that was added to board/cr50/build.mk:

# Add hardware crypto support.
PDIR=private-cr51

well, I am not sure what is the best way to do it then - I would rather not exclude cr50 from makeall unconditionally.

Can the public contributors be educated to remove cr50 directory before running makeall?

> Can the public contributors be educated to remove cr50 directory before running makeall?

Yes, but it's an inconvenience, and it seems like a generally unfriendly thing to do, if we take cros-ec seriously as an open source project.

Can cr50 be made entirely private, or can it be reworked to still build even if private-cr51 is absent?

In fact there are plans of making cr50 entirely public.

Permanently excluding it from makeall is not a good idea as this would let build breakages creep in.

Let's land https://chromium-review.googlesource.com/#/c/354540/ (or similar) which will exclude cr50 from `make buildall` when private-cr51 is not present. Then, we can revert https://chromium-review.googlesource.com/350095 and correctly fail in case cr50 adds more private dependencies later.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromiumos/platform/ec/+/58c0727901aae21c496056a5eccaa27c693510ef

commit 58c0727901aae21c496056a5eccaa27c693510ef
Author: Shawn Nematbakhsh <shawnn@chromium.org>
Date: Tue Jun 21 19:37:01 2016

build: Skip cr50 board in `make buildall` if private folder not present

BUG= chromium:621993 
BRANCH=None
TEST=`make buildall -j` from public checkout succeeds. Also verify cr50
still built from private checkout.

Change-Id: I982806e282146aab76154b51c366226d3d1aed14
Signed-off-by: Shawn Nematbakhsh <shawnn@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/354540
Commit-Ready: Shawn N <shawnn@chromium.org>
Tested-by: Shawn N <shawnn@chromium.org>
Reviewed-by: Vadim Bendebury <vbendeb@chromium.org>

[modify] https://crrev.com/58c0727901aae21c496056a5eccaa27c693510ef/Makefile.rules

The following revision refers to this bug:
  https://chromium.googlesource.com/chromiumos/overlays/chromiumos-overlay/+/95147e4c4d36c69f5bd61fb548b429712ec71052

commit 95147e4c4d36c69f5bd61fb548b429712ec71052
Author: Shawn N <shawnn@chromium.org>
Date: Wed Jun 22 01:39:58 2016

Revert "chromeos-ec: do not build cr50 when making buildall"

CL:354540 skips building cr50 when dependent private folders are not
present, so this ebuild modification should no longer be necessary to
pass `make buildall` on public builders.

BUG= chromium:621993 
BRANCH=None
TEST=`emerge-glados chromeos-ec` from public checkout,
verify no failure.

Change-Id: Ic7a5dba5a6f447639dcd1a58b0e640d31f03d5a4
Reviewed-on: https://chromium-review.googlesource.com/354771
Commit-Ready: Shawn N <shawnn@chromium.org>
Tested-by: Shawn N <shawnn@chromium.org>
Reviewed-by: Vadim Bendebury <vbendeb@chromium.org>

[modify] https://crrev.com/95147e4c4d36c69f5bd61fb548b429712ec71052/chromeos-base/chromeos-ec/chromeos-ec-9999.ebuild

Bulk verified