sdk: rename README + update Makefile

'help' target fails not finding a file, so follow up on a change[2] made
as a fix for main README[1].

1. d0113711a3 ("README: port to 21st century")
2. 751486b31f ("build: fix README.md reference after rename")

Signed-off-by: Tomasz Maciej Nowak <tmn505@gmail.com>
(cherry picked from commit 2d5f7035cf)
(cherry picked from commit e9911f10e4)
Signed-off-by: Christian Lamparter <chunkeey@gmail.com>

target/sdk/Makefile

@@ -147,7 +147,7 @@ $(BIN_DIR)/$(SDK_NAME).tar.xz: clean
 		./files/Config.in \
 		./files/Makefile \
 		./files/include/prepare.mk \
-		./files/README.SDK \
+		./files/README.md \
 		$(SDK_BUILD_DIR)/
 	mkdir -p $(SDK_BUILD_DIR)/package/kernel
 	$(CP) \