Makefile: make GNU_HOST_NAME a simply expanded variable (:=)

GNU_HOST_NAME is referred more than 100 times in the tree, used 5
times during startup, and is relatively expensive to run - so make
it a simple expanded variable (:=) rather than recursively expanded
one (=).

Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>

Makefile

@@ -153,7 +153,7 @@ cc-option=$(shell if $(TARGET_CC) $(TARGET_CFLAGS) $(1) -S -o /dev/null -xc /dev
 unexport CROSS_COMPILE
 unexport ARCH
 
-GNU_HOST_NAME=$(shell package/gnuconfig/config.guess)
+GNU_HOST_NAME:=$(shell package/gnuconfig/config.guess)
 
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