kconfig: Only generate config_is_xxx for bool and tristate options

For strings and integers, the config_is_xxx macros are useless and
sometimes misleading:

  #define CONFIG_INITRAMFS_SOURCE ""
  #define config_is_initramfs_source() 1

Cc: Jean-Christophe PLAGNIOL-VILLARD <plagnioj@jcrosoft.com>
Signed-off-by: Michal Marek <mmarek@suse.cz>