build: implement make check and make package/X/check

This is intended to be used for a wide array of package sanity checks.

The first check that is implemented is for the hash of downloaded files.
It checks:
  - Missing hash
  - Use of SHA256 instead of MD5
  - dl/<file> hash not matching hash in makefile
  - deprecated MD5SUM variable

The deprecated MD5SUM variable check is skipped for feeds/ until OpenWrt
is updated as well

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

rules.mk

@@ -15,6 +15,11 @@ endif
 include $(TOPDIR)/include/debug.mk
 include $(TOPDIR)/include/verbose.mk
 
+ifneq ($(filter check,$(MAKECMDGOALS)),)
+CHECK:=1
+DUMP:=1
+endif
+
 export TMP_DIR:=$(TOPDIR)/tmp
 
 qstrip=$(strip $(subst ",,$(1)))
@@ -386,6 +391,9 @@ all:
 FORCE: ;
 .PHONY: FORCE
 
+check: FORCE
+	@true
+
 val.%:
 	@$(if $(filter undefined,$(origin $*)),\
 		echo "$* undefined" >&2, \