buildman: Remove output binaries before building

Buildman reuses build directories from previous builds to avoid the cost
of 'make mrproper' for every build. If the previous build produced an SPL
image but the current one does not, the SPL image will remain and buildman
will think it is a result of building the current board.

Remove these files before building, to avoid this problem.

Signed-off-by: Simon Glass <sjg@chromium.org>