cr50 release: use signer executable installed on chroot
cr50 release: use signer executable installed on chroot

A new ebuild allows to install codesigner as /usr/bin/cr50-codesigner,
let's make use of it instead of manually copied instance of the
signer.

BRANCH=none
BUG=b:74100307
TEST=verified that error message is generated if cr50-codesigner is
     not installed, and that signing succeeds once cr50-codesigner is
     installed.

Change-Id: I468803443e7b052a8ecb074ee80f63f588888985
Signed-off-by: Vadim Bendebury <5515d6d2d0829cbe0dd0dcf2094aaded06d58514@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/982495
Reviewed-by: Mary Ruthven <81380f3403fed74252e92df3ed012a1819ec6296@chromium.org>

coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec