rules.mk: reduce shexport to one line to allow it to be put in a target context

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

SVN-Revision: 43018

rules.mk

@@ -278,8 +278,7 @@ V_$(subst .,_,$(subst -,_,$(subst /,_,$(1))))
 endef
 
 define shexport
-$(call shvar,$(1))=$$(call $(1))
-export $(call shvar,$(1))
+export $(call shvar,$(1))=$$(call $(1))
 endef
 
 define include_mk