Makefile: use make builtin rather than shell
Makefile: use make builtin rather than shell

Since READLINK(1) varies across platforms, namely darwin version does
not have the `-f` flag, then don't rely on the shell. Especially since
gnu make already has this as a builtin function.

Signed-off-by: Vincent Batts <d01a4772f0cffd3f005a7d7f01c59cd104c7a5fe@hashbangbash.com>

opencontainers/image-spec,jonboulle/image-spec,jonboulle/image-spec,opencontainers/image-spec