gitignore: Put build-*/ pattern in top-level gitignore file.

.gitignore

@@ -20,9 +20,10 @@
 ######################
 *.swp
 
-# Build directory
+# Build directories
 ######################
 build/
+build-*/
 
 # Test failure outputs
 ######################

ports/nrf/.gitignore

 # Nordic files
 #####################
 drivers/bluetooth/s1*/
-
-# Build files
-#####################
-build-*/
-

ports/stm32/.gitignore

-build-*/

ports/unix/.gitignore

-build
-build-fast
-build-minimal
-build-coverage
-build-nanbox
-build-freedos
 micropython
 micropython_fast
 micropython_minimal