kbuild: simplify build, clean, modbuiltin shorthands

$(if $(KBUILD_SRC),$(srctree)/) was a useful strategy
to omit a long absolute path for in-source-tree build
prior to commit 890676c65d699db3ad82e7dddd0cf8fb449031af
(kbuild: Use relative path when building in the source tree).

Now $(srctree) is "." when building in the source tree.
It would not be annoying to add "$(srctree)/" all the time.

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
Signed-off-by: Michal Marek <mmarek@suse.cz>
diff --git a/scripts/Kbuild.include b/scripts/Kbuild.include
index 8a9a4e1..65e7b08 100644
--- a/scripts/Kbuild.include
+++ b/scripts/Kbuild.include
@@ -171,13 +171,13 @@
 # Shorthand for $(Q)$(MAKE) -f scripts/Makefile.build obj=
 # Usage:
 # $(Q)$(MAKE) $(build)=dir
-build := -f $(if $(KBUILD_SRC),$(srctree)/)scripts/Makefile.build obj
+build := -f $(srctree)/scripts/Makefile.build obj
 
 ###
 # Shorthand for $(Q)$(MAKE) -f scripts/Makefile.modbuiltin obj=
 # Usage:
 # $(Q)$(MAKE) $(modbuiltin)=dir
-modbuiltin := -f $(if $(KBUILD_SRC),$(srctree)/)scripts/Makefile.modbuiltin obj
+modbuiltin := -f $(srctree)/scripts/Makefile.modbuiltin obj
 
 # Prefix -I with $(srctree) if it is not an absolute path.
 # skip if -I has no parameter