packages: apply usign padding workarounds to package indexes if needed

Since usign miscalculates SHA-512 digests for input sizes of exactly
64 + N * 128 + 110 or 64 + N * 128 + 111 bytes, we need to apply some
white space padding to avoid triggering the hashing edge case.

While usign itself has been fixed already, there is still many firmwares
in the wild which use broken usign versions to verify current package
indexes so we'll need to carry this workaround in the forseeable future.

Ref: https://forum.openwrt.org/t/signature-check-failed/41945
Ref: https://git.openwrt.org/5a52b379902471cef495687547c7b568142f66d2
Signed-off-by: Jo-Philipp Wich <jo@mein.io>
(backported from commit e1f588e446)

package/Makefile

@@ -80,8 +80,12 @@ $(curdir)/index: FORCE
 		mkdir -p $$d; \
 		cd $$d || continue; \
 		$(SCRIPT_DIR)/ipkg-make-index.sh . 2>&1 > Packages.manifest; \
-		grep -vE '^(Maintainer|LicenseFiles|Source|Require)' Packages.manifest > Packages && \
-			gzip -9nc Packages > Packages.gz; \
+		grep -vE '^(Maintainer|LicenseFiles|Source|Require)' Packages.manifest > Packages; \
+		case "$$(((64 + $$(stat -L -c%s Packages)) % 128))" in 110|111) \
+			$(call ERROR_MESSAGE,WARNING: Applying padding in $$d/Packages to workaround usign SHA-512 bug!); \
+			{ echo ""; echo ""; } >> Packages;; \
+		esac; \
+		gzip -9nc Packages > Packages.gz; \
 	); done
 ifdef CONFIG_SIGNED_PACKAGES
 	@echo Signing package index...