kbuild: set correct abs_srctree and abs_objtree for package builds

When you run 'make rpm-pkg', the rpmbuild tool builds the kernel in
rpmbuild/BUILD, but $(abs_srctree) and $(abs_objtree) point to the
directory path where make was started, not the kernel is actually
being built. The same applies to 'make snap-pkg'. Fix it.
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>

Makefile

@@ -38,6 +38,10 @@ __all:
 # descending is started. They are now explicitly listed as the
 # prepare rule.
 
+this-makefile := $(lastword $(MAKEFILE_LIST))
+export abs_srctree := $(realpath $(dir $(this-makefile)))
+export abs_objtree := $(CURDIR)
+
 ifneq ($(sub_make_done),1)
 
 # Do not use make's built-in rules and variables
@@ -185,8 +189,6 @@ $(if $(abs_objtree),, \
 
 # $(realpath ...) resolves symlinks
 abs_objtree := $(realpath $(abs_objtree))
-else
-abs_objtree := $(CURDIR)
 endif # ifneq ($(KBUILD_OUTPUT),)
 
 ifeq ($(abs_objtree),$(CURDIR))
@@ -196,9 +198,6 @@ else
 need-sub-make := 1
 endif
 
-this-makefile := $(lastword $(MAKEFILE_LIST))
-abs_srctree := $(realpath $(dir $(this-makefile)))
-
 ifneq ($(words $(subst :, ,$(abs_srctree))), 1)
 $(error source directory cannot contain spaces or colons)
 endif
@@ -211,7 +210,6 @@ need-sub-make := 1
 $(this-makefile): ;
 endif
 
-export abs_srctree abs_objtree
 export sub_make_done := 1
 
 ifeq ($(need-sub-make),1)