luci.mk: fix length of abbreviated hash in version

luci.mk: fix length of abbreviated hash in version

Fix the length of the abbreviated commit hash used in
the LuCI version string used in the package names.
Use the old standard of seven characters in hash.

New git versions vary the length of the abbreviated
commit hash based on the number of objects in repo. That
may lead into different hash lengths with e.g. having
full/shallow git history, having all/single branch or
having different git versions in different build hosts.

Different hash lengths in package name may lead to
sneaky opkg install errors, as opkg apparently preserves
info for removed packages and may prefer that.
Example:
$ opkg install http://downloads...luci-app-statistics_git-17.073.42825-b47a21f-1_all.ipk
Downloading http://downloads...luci-app-statistics_git-17.073.42825-b47a21f-1_all.ipk
Installing luci-app-statistics (git-17.073.42825-b47a21fbb-1) to root...
Collected errors:
 * opkg_download_pkg: Package luci-app-statistics is not available from any configured src.

The error above is ultimately in opkg, but mitigate
the problem by fixing the hash length when building LuCI.

Signed-off-by: Hannu Nyman <hannu.nyman@iki.fi>