util/lint: Show an error if a symbol is created in two choice blocks

Kconfig shows a warning about this, but we want to catch it earlier
and halt the build.

Change-Id: I0acce1d40a6ca2b212c638bdb1ec65de5bd4d726
Signed-off-by: Martin Roth <martinroth@google.com>
Reviewed-on: https://review.coreboot.org/18970
Tested-by: build bot (Jenkins)
Reviewed-by: Subrata Banik <subrata.banik@intel.com>
Reviewed-by: Patrick Georgi <pgeorgi@google.com>
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>

util/lint/kconfig_lint_README

@@ -68,7 +68,8 @@ Errors in Kconfig files:
 - Choice block defined with no symbols.
 - The 'tristate' type is not used in coreboot.
 - A 'select' keyword used outside of a config block.
-- Symbols created both inside and outside of a choice block.
+- Symbols created both inside and outside of a choice block or in two
+  different choice blocks.
 - A 'range' keyword has higher minimum than maximum value.
 - A config block with a prompt at the top level (the top level is currently
   just for menus).