Build: Fix parallel building

Build: Fix parallel building

Soren reports build fails if -j option is given:

  $ make -j16 CROSS_COMPILE=aarch64-linux-gnu-
  Building fvp
  make: *** No rule to make target 'build/fvp/release/bl1/',
                    needed by 'build/fvp/release/bl1/bl1.ld'.  Stop.
  make: *** Waiting for unfinished jobs....

The cause of the failure is that $(dir ) leaves a trailing / on the
directory names.   It must be ripped off to let Make create the
directory.

There are some ways to fix the issue.  Here, I chose to make MAKE_LD
look like MAKE_C and MAKE_S because bl*_dirs seems the central place
of making directories.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Reported-by: Soren Brinkmann <soren.brinkmann@xilinx.com>
Tested-by: Soren Brinkmann <soren.brinkmann@xilinx.com>