kbuild: use $(abspath ...) instead of $(shell cd ... && /bin/pwd)

Kbuild conventionally uses $(shell cd ... && /bin/pwd) idiom to get
the absolute path of the directory because GNU Make 3.80, the minimal
supported version at that time, did not support $(abspath ...) or
$(realpath ...).

Commit 37d69ee308 ("docs: bump minimal GNU Make version to 3.81")
dropped the GNU Make 3.80 support, so we are now allowed to use those
make-builtin helpers.

This conversion will provide better portability without relying on
the pwd command or its location /bin/pwd.

I am intentionally using $(realpath ...) instead $(abspath ...) in
some places.  The difference between the two is $(realpath ...)
returns an empty string if the given path does not exist.  It is
convenient in places where we need to error-out if the makefile fails
to create an output directory.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Acked-by: Thierry Reding <treding@nvidia.com>

tools/power/cpupower/Makefile

@@ -26,7 +26,7 @@ endif
 
 ifneq ($(OUTPUT),)
 # check that the output directory actually exists
-OUTDIR := $(shell cd $(OUTPUT) && /bin/pwd)
+OUTDIR := $(realpath $(OUTPUT))
 $(if $(OUTDIR),, $(error output directory "$(OUTPUT)" does not exist))
 endif