linux: add linux-update-config and linux-update-defconfig targets

In analogy to build targets uclibc-update and busybox-update, add
extra targets to copy the current configuration to the custom
configuration file set in buildroot. These targets facilitate the
work of developers adding support for a specific board.

linux-update-config copies .config directly.
linux-update-defconfig first creates a defconfig and copies that.

Signed-off-by: Thomas De Schampheleire <thomas.de.schampheleire@gmail.com>
Acked-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>