kconfig: resort the documentation of the environment variables

All the KCONFIG_ environment variables were previously located in a
section "Environment variables in 'menuconfig'", but neither are they
restricted to 'menuconfig' nor are they all used by 'menuconfig'.

Introduce the following three sections for these variables:
  * Environment variables for '*config'
  * Environment variables for '{allyes/allmod/allno/rand}config'
  * Environment variables for 'silentoldconfig'

Furthermore this puts MENUCONFIG_MODE next to MENUCONFIG_COLOR into a
common section "User interface options for 'menuconfig'".

Signed-off-by: Markus Heidelberg <markus.heidelberg@web.de>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>