Added package GnuPG (GPG) to the build

From the make menuconfig help:

BR2_PACKAGE_GNUPG:

GnuPG is the GNU project's complete and free implementation of the OpenPGP
standard as defined by RFC4880. GnuPG allows to encrypt and sign your data and
communication, features a versatile key management system as well as access
modules for all kinds of public key directories. GnuPG, also known as GPG, is a
command line tool with features for easy integration with other applications.

http://gnupg.org/

Symbol: BR2_PACKAGE_GNUPG [=y]
Type  : boolean
Prompt: gnupg
  Defined at package/gnupg/Config.in:1
  Location:
    -> Package Selection for the target
      -> Shell and utilities
  Selects: BR2_PACKAGE_ZLIB [=y] && BR2_PACKAGE_NCURSES [=y]
Enabled RSA feature in the GnuPG package

In order to do that I had to apply a patch from the main buildroot repo: commit
number b2fab93b16.

Description from the buildroot repo:

   gnupg: option to include RSA support.

   gnupg is compiled with --enable-minimal flag. This produces a binary that
   only supports ElGamal and DSA public key algorithms.

   RSA has been the default for `gpg --gen-key` since 2009, so it makes sense to
   be able to build a gnupg binary that supports it.

   Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

Commit URL:

http://git.buildroot.net/buildroot/commit/?id=b2fab93b1636027c73c664cdf395cb8f93dc55dc
Added a `full version' config option to GnuPG