kbuild: revert $(realpath ...) to $(shell cd ... && /bin/pwd)

I thought commit 8e9b4667 ("kbuild: use $(abspath ...) instead of
$(shell cd ... && /bin/pwd)") was a safe conversion, but it changed
the behavior.

$(abspath ...) / $(realpath ...) does not expand shell special
characters, such as '~'.

Here is a simple Makefile example:

  ---------------->8----------------
  $(info /bin/pwd: $(shell cd ~/; /bin/pwd))
  $(info abspath: $(abspath ~/))
  $(info realpath: $(realpath ~/))
  all:
          @:
  ---------------->8----------------

  $ make
  /bin/pwd: /home/masahiro
  abspath: /home/masahiro/workspace/~
  realpath:

This can be a real problem if 'make O=~/foo' is invoked from another
Makefile or primitive shell like dash.

This commit partially reverts 8e9b4667.

Fixes: 8e9b4667 ("kbuild: use $(abspath ...) instead of $(shell cd ... && /bin/pwd)")
Reported-by: Julien Grall <julien.grall@arm.com>
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Tested-by: Julien Grall <julien.grall@arm.com>