image.in/functions.mk: added ARCH dup

If the build is (re-)run withing generated profile directory,
the makefiles no longer have ARCH passed by upstream -- the
upstream might be unreachable at that point indeed.

This is unfortunate but we should cope with that.

NB: might be revisited when architecture-specific packagelists emerge
since currently there's no difference in package lists so we can just
re-run the build for a different architecture but this might change...

image.in/functions.mk

@@ -1,5 +1,8 @@
 # globals
 PKGDIR := $(GLOBAL_BUILDDIR)/pkg
+
+### duplicated from toplevel build.mk, log.mk for sake of "local" builds
+ARCH ?= $(shell arch | sed 's/i686/i586/')
 DATE ?= $(shell date +%Y%m%d)
 
 # prefix pkglist name with its directory to form a path