cmake: fix bug introduced with `PRODUCT_BUILD_DIR` implementation

The recent change to use `DIRECT_BUILD` introduced an issue where
the `BUILD_PATH` is not being updated as intended. This fix ensures
`PRODUCT_BUILD_DIR` is correctly set and used throughout the
Makefile wrapper.

Signed-off-by: Leandro Belli <leandro.belli@arm.com>
Change-Id: I4551fda576b9dedad33dd4be540dce687aa5bba1