mk: allow renaming of build directories

mk: allow renaming of build directories

When building using make, the Makefile in the build directory contained
the name of the build directory to be passed as an "O=" parameter to
the DPDK SDK makefiles. Unfortunately, this meant that the compilation
would always fail if the build directory was renamed. To remove this
limitation, we can use $(CURDIR) instead of the directory name.

Signed-off-by: Bruce Richardson <bruce.richardson@intel.com>