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>
package/community-profiles: fix netmask for freifunk gadow
httpclient: fix logic error for redirects
fixes missing imports for gwcheck

since r41281 (83e9122) network.sh depends on jsonfilter instead of jshn.
Therefor jshn is missing for gwcheck and needs to be sourced manually.
Freifunk: correct bbsid in Potsdam on channel 5
modules/base: attempt to work around Firefox autocomplete bugs

Ref: https://dev.openwrt.org/ticket/18176

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