initscript functions: add readhash

initscript functions: add readhash

To avoid the usage of eval and to store the config in an key value
array, we introduce an new function. The tests only check if we
read the correct value to the correct variable.

One comment on the implementation as this has created some headache:

>From https://www.gnu.org/software/bash/manual/bash.html#Bourne-Shell-Builtins

	"When used in a function, declare makes each name local, as with the local command, unless the -g option is used."

So we need to use -g here

Signed-off-by: Jonatan Schlag <jonatan.schlag@ipfire.org>
Signed-off-by: Michael Tremer <michael.tremer@ipfire.org>