Add "git describe" based version string postfix

Add "git describe" based version string postfix

If hostapd or wpa_supplicant is built from a git repository, add a
VERSION_STR postfix from the current git branch state. This is from "git
describe --dirty=+". VERSION_STR will thus look something like
"2.6-devel-hostap_2_5-132-g4363c0d+" for development builds from a
modified repository.

This behavior is enabled automatically if a build within git repository
is detected (based on ../.git existing). This can be disabled with
CONFIG_NO_GITVER=y in wpa_supplicant/.config and hostapd/.config.

Signed-off-by: Jouni Malinen <j@w1.fi>