build: generate strictly increasing git revisions (#225)

Instead of simply emitting a "git-$hash" revision, take the commit time into
account to generate strictly increasing revisions to preserve opkg upgrade
capabilities.

The format is "git-YY.DDD.SSSSS-HHHHHHH" with the following defined fields:

  - YY        year number (00..99)
  - DDD       day of year (001..366)
  - SSSSSS    second of day (00000..86399)
  - HHHHHHH   abbreviated git commit hash

An example revision is "git-14.283.50202-bb388f0".

Signed-off-by: Jo-Philipp Wich <jow@openwrt.org>