toolchain: binutils: simplify patch management

toolchain: binutils: simplify patch management

Take in account only first two version components to lookup patch directory.
Hovewer, computed "BASE_VERSION" may be overridden (if necessary).
This change should prevent further issues with binutils being unpatched, see commits adad973a9c34 and 525a1e94b343.

Also drop obsolete "BIN_VERSION" variable (not used anywhere).

Signed-off-by: Konstantin Demin <rockdrilla@gmail.com>
Link: https://github.com/openwrt/openwrt/pull/21997
Signed-off-by: Nick Hainke <vincent@systemli.org>