[openwrt/openwrt] sdk: rename README + update Makefile

[LEDE Commits]

chunkeey pushed a commit to openwrt/openwrt.git, branch openwrt-23.05:
https://git.openwrt.org/b52cfba97ab708906fcbf36bba062e7c2e0f07dd

commit b52cfba97ab708906fcbf36bba062e7c2e0f07dd
Author: Tomasz Maciej Nowak <tmn505 at gmail.com>
AuthorDate: Tue Jul 11 16:09:49 2023 +0200

    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. d0113711a31f ("README: port to 21st century")
    2. 751486b31fd9 ("build: fix README.md reference after rename")
    
    Signed-off-by: Tomasz Maciej Nowak <tmn505 at gmail.com>
    (cherry picked from commit 2d5f7035cf45801158bed6f5d0ac0de0002c1810)
    (cherry picked from commit e9911f10e482f3174f745a36c0c9fd7964758caf)
    Signed-off-by: Christian Lamparter <chunkeey at gmail.com>
---
 target/sdk/Makefile                        | 2 +-
 target/sdk/files/{README.SDK => README.md} | 0
 2 files changed, 1 insertion(+), 1 deletion(-)

diff --git a/target/sdk/Makefile b/target/sdk/Makefile
index b688eb4cba..667d00b43f 100644
--- a/target/sdk/Makefile
+++ b/target/sdk/Makefile
@@ -145,7 +145,7 @@ $(BIN_DIR)/$(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) \
diff --git a/target/sdk/files/README.SDK b/target/sdk/files/README.md
similarity index 100%
rename from target/sdk/files/README.SDK
rename to target/sdk/files/README.md