Makefile.target: set master BUILD_DIR

Makefile.target: set master BUILD_DIR

make can be invoked in the individual build dirs to build an individual
target or just a single file of a target. e.g.

touch translate-all.c
make -C microblazeel-softmmu translate-all.o

There is however a small bug when using the pixman submodule.
config-host.mak will ref BUILD_DIR for the pixman -I CFLAGS:

grep BUILD_DIR config-host.mak
QEMU_CFLAGS=-I$(SRC_PATH)/pixman/pixman -I$(BUILD_DIR)/pixman/pixman ...

This causes a build failure as -I/pixman/pixman (BUILD_DIR=="") will
not be found.

BUILD_DIR is usually set by the top level Makefile. Just lazy-set it in
Makefile.target to the parent directory.

Granted, this will not work if the pixman submodule is not prebuilt,
but it at least means you can do incremental partial builds once you
have done your initial full build (or attempt) from the top level.

The next step would be refactor make infrastructure to rebuild pixman
on a submake like the one above.

Cc: Gerd Hoffmann <kraxel@redhat.com>
Signed-off-by: Peter Crosthwaite <crosthwaite.peter@gmail.com>
Message-Id: <1432618686-16077-1-git-send-email-crosthwaite.peter@gmail.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>