kbuild: hardcode genksyms path and remove GENKSYMS variable

Back port from kernel although it's not used since it makes diffing
easier.
commit 88110713ca9dfb ("kbuild: hardcode genksyms path and remove GENKSYMS variable")

Signed-off-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>
diff --git a/scripts/Makefile.build b/scripts/Makefile.build
index 2b2beb0..b7611c0 100644
--- a/scripts/Makefile.build
+++ b/scripts/Makefile.build
@@ -148,7 +148,7 @@
 
 cmd_gensymtypes =                                                           \
     $(CPP) -D__GENKSYMS__ $(c_flags) $< |                                   \
-    $(GENKSYMS) $(if $(1), -T $(2))                                         \
+    scripts/genksyms/genksyms $(if $(1), -T $(2))			    \
      $(if $(KBUILD_PRESERVE),-p)                                            \
      -r $(firstword $(wildcard $(2:.symtypes=.symref) /dev/null))