kconfig-package: add support for config fragments

Adds functionality to the kconfig infrastructure to merge additional
configuration fragment files to the main configuration file of
kconfig packages, using support/kconfig/merge_config.sh

Typical use-case is when you want your configuration to be
kept in sync with an upstream (def)config file, but do require
some minor local modifications.

Disables -update-config and -update-defconfig targets when
fragment files are set.

[Thomas: take into account comments made by Arnout:
  - Minor fixes in the documentation changes
  - Add @ before the tests done in the $(1)-update-config and
    $(1)-update-defconfig targets.]

Signed-off-by: Floris Bos <bos@je-eigen-domein.nl>
Tested-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Tested-by: Gergely Imreh <imrehg@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>