Add generic functions to enable/set/disable options in kconfig files

The KCONFIG_ENABLE_OPT, KCONFIG_SET_OPT and KCONFIG_DISABLE_OPT are
new make functions to respectively enable, set and disable options in
Kconfig-like files (as used by the kernel, uClibc or Busybox).

They can be used as follows :

 $(call KCONFIG_ENABLE_OPT,CONFIG_FOOBAR,/path/to/.config)
 $(call KCONFIG_SET_OPT,CONFIG_BARFOO,foobar,/path/to/.config)
 $(call KCONFIG_DISABLE_OPT,CONFIG_FARBOO,/path/to/.config)

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>