kbuild: /bin/pwd -> pwd

Most places use pwd and rely on $PATH lookup. Moving the remaining
absolute path /bin/pwd users over for consistency.

Also, a reason for doing /bin/pwd -> pwd instead of the other way around
is because I believe build systems should make little assumptions on
host filesystem layout. Case in point, we do this kind of patching
already in NixOS.

Ref. commit 028568d8


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

Signed-off-by: Bjørn Forsman <bjorn.forsman@gmail.com>
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>