core: include arch-specific definitions

Allow architectures to define variables and helper macros.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Cc: Peter Korsgaard <jacmet@uclibc.org>
Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>

Makefile

@@ -479,6 +479,9 @@ all: world
 include Makefile.legacy
 
 include package/Makefile.in
+# arch/arch.mk.* must be after package/Makefile.in because it may need to
+# complement variables defined therein, like BR_NO_CHECK_HASH_FOR.
+-include $(wildcard arch/arch.mk.*)
 include support/dependencies/dependencies.mk
 
 include toolchain/*.mk