Makefile: Handle building in a very old build directory

Versions of U-Boot before 2014.01 created a symlink from include/asm to
the architecture-specific header directory.

If an ARM board is build with that old version, then sandbox is built on
a more recent version (both with in-tree builds), the include/asm symlink
confuses the build system. It picks up the ARM headers when it should be
using the sandbox ones.

Since 2014 U-Boot has only created a symlink inside the include/asm/
directory and only for out-of-tree builds. So for in-tree builds it does
not expect to see an include/asm symlink. It is not removed by
'make mrproper'. It does show up with 'git status' but is easy enough to
miss.

Add include/asm to the files to remove with 'make mkproper'. For recent
U-Boot builds this has no effect, since include/asm is a directory, not a
file. If the include/asm symlink is there, it will be removed.

Reported-by: Marek Szyprowski <m.szyprowski@samsung.com>
Signed-off-by: Simon Glass <sjg@chromium.org>