egencache: add a --repos-conf option

Passing large repos.conf as inline strings can be very spammy.
Add an option to point to a custom repos.conf file instead.

BUG=b:244220188
TEST=`egencache --update --repos-conf /tmp/repos.conf --repo baseboard-asurada` works

Change-Id: Idd3847c77b25496addfcf3ca1befb354ac95b94f
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/third_party/portage_tool/+/3864250
Reviewed-by: Lizzy Presland <zland@google.com>
Auto-Submit: Mike Frysinger <vapier@chromium.org>
Commit-Queue: Lizzy Presland <zland@google.com>
Tested-by: Mike Frysinger <vapier@chromium.org>
2 files changed