Makefile: bugfix for handling platform paths

If the path where this repo is located contains the platform name on
it, the original Makefile replaced its occurrences from the path making
it an invalid path. This commit prevents this behavior replacing only
the last part of the path as intended.

Signed-off-by: Alejandro Cabrera Aldaya <aldaya@gmail.com>
Reviewed-by: Xiang W <wxjstz@126.com>
Reviewed-by: Andrew Jones <ajones@ventanamicro.com>
Reviewed-by: Anup Patel <anup@brainfault.org>