fix(build): Fix for proper string variable check.

* Properly load variable for comparison for shield name substring
  calculations.

app/cmake/ZephyrBuildConfig.cmake

@@ -122,7 +122,7 @@ if(DEFINED SHIELD)
 		while(NOT S_PIECES STREQUAL "")
 			list(POP_BACK S_PIECES)
 			list(JOIN S_PIECES "_" s_substr)
-			if ("%{s_substr}" STREQUAL "" OR "${s_substr}" STREQUAL ${shield_dir_name})
+			if ("${s_substr}" STREQUAL "" OR "${s_substr}" STREQUAL "${shield_dir_name}")
 				break()
 			endif()
 			list(APPEND shield_candidate_names ${s_substr})