cmake: sort shield list and improve indent

Currently, when usage.cmake is run, the shield list is in construction
order, making it difficult to locate the shield of interest. This sorts
the list upon display and also makes the display indent consistent.

Signed-off-by: Aleksandr Koltsoff <czr@iki.fi>

cmake/usage/usage.cmake

@@ -49,8 +49,10 @@ dump_all_boards("" "  ")
 message("")
 message("Supported Shields:")
 message("")
-foreach(shield ${SHIELD_LIST})
-  message(" ${shield}")
+set(sorted_shield_list ${SHIELD_LIST})
+list(SORT sorted_shield_list)
+foreach(shield ${sorted_shield_list})
+  message("  ${shield}")
 endforeach()
 message("")
 message("Build flags:")