build: add a small standalone utility for calculating md5/sha256 hash

This will be used to simplify the build system code for checking hashes.
Instead of using various variants of md5sum / openssl, use one simple
utility for all of them

Signed-off-by: Felix Fietkau <nbd@nbd.name>

include/prereq-build.mk

@@ -156,6 +156,12 @@ $(eval $(call SetupHostCommand,git,Please install Git (git-core) >= 1.7.12.2, \
 $(eval $(call SetupHostCommand,file,Please install the 'file' package, \
 	file --version 2>&1 | grep file))
 
+$(STAGING_DIR_HOST)/bin/mkhash: $(SCRIPT_DIR)/mkhash.c
+	mkdir -p $(dir $@)
+	$(CC) -O2 -I$(TOPDIR)/tools/include -o $@ $<
+
+prereq: $(STAGING_DIR_HOST)/bin/mkhash
+
 # Install ldconfig stub
 $(eval $(call TestHostCommand,ldconfig-stub,Failed to install stub, \
 	touch $(STAGING_DIR_HOST)/bin/ldconfig && \