initscripts: readhash: Fix handling = signs

initscripts: readhash: Fix handling = signs

The function expected that a line only contains exactly one equals sign
(=) which is not fit for purpose. In the WireGuard code we hold key
material that is encoded in base64 and therefore contains padding that
uses =.

This patch fixes that we expect exactly one equals sign immediately
after the key and we will then accept more = in the value - which was
already permitted.

Furthermore, this patch fixes the splitting if the key and value at the
first =.

Signed-off-by: Michael Tremer <michael.tremer@ipfire.org>
Signed-off-by: Arne Fitzenreiter <arne_f@ipfire.org>