kbuild: avoid environment to set variables used by kbuild

A few of the variables used by kbuild has fixed naming.
Make sure we do not pick up random values from the environment.
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>

scripts/Makefile.build

@@ -7,6 +7,22 @@ src := $(obj)
 PHONY := __build
 __build:
 
+# Init all relevant variables used in kbuild files so
+# 1) they have correct type
+# 2) they do not inherit any value from the environment
+obj-y :=
+obj-m :=
+lib-y :=
+lib-m :=
+always :=
+targets :=
+subdir-y :=
+subdir-m :=
+EXTRA_AFLAGS   :=
+EXTRA_CFLAGS   :=
+EXTRA_CPPFLAGS :=
+EXTRA_LDFLAGS  :=
+
 # Read .config if it exist, otherwise ignore
 -include include/config/auto.conf