build: let make check warn about use of legacy PKG_MD5SUM variable in feeds

The variable rename change has been merged in OpenWrt now, so it's
possible to convert the feeds as well.

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

include/download.mk

@@ -94,13 +94,10 @@ endif
 
 C_md5_deprecated = Use of $(2) is deprecated, switch to $(3)
 
-# Skip MD5SUM check in feeds until OpenWrt is updated
-ifneq ($(filter $(foreach dir,package tools toolchain, $(TOPDIR)/$(dir)/%),$(CURDIR)),)
 check_md5 = \
   $(if $(filter-out x,$(1)), \
     $(call check_warn,md5_deprecated,$(1),$(2),$(3)) \
   )
-endif
 
 hash_var = $(if $(filter-out x,$(1)),MD5SUM,HASH)
 endif