packages infra: add function to get a Kconfig option

We so far have no mean to get the value from a Kconfig option from the
.config file of a package (eg. linux, busybox...).

Add a new function that returns the unmangled value of an option.
It expect two arguments:
  - the Kconfig option name (complete, with leading CONFIG if necessary)
  - the .config file to get it from

Note that, if the Kconfig option is a string, the returned value will
contain the leading and trailing double-quotes.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Peter Korsgaard <jacmet@uclibc.org>
Cc: Arnout Vandecappelle <arnout@mind.be>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>