rtai: remove option BR2_LINUX_KERNEL_EXT_RTAI_PATCH

rtai: remove option BR2_LINUX_KERNEL_EXT_RTAI_PATCH

This commit removes BR2_LINUX_KERNEL_EXT_RTAI_PATCH because this
option never worked. It was added in commit
8797a9cd1fe6723db34b0c125d0d9d04e3483e8d, which added package/rtai/
and RTAI as a Linux extension.

The option prompt says "Path for RTAI patch file", so let's say you
specify /home/foo/bar/myrtai.patch as the value for
BR2_LINUX_KERNEL_EXT_RTAI_PATCH.

Then the code does:

RTAI_PATCH = $(call qstrip,$(BR2_LINUX_KERNEL_EXT_RTAI_PATCH))

and we have a package called 'rtai', so the normal logic of
<pkg>_PATCH applies. Since the <pkg>_PATCH value does not contain
ftp://, http:// or https://, the package infrastructure will try to
download $(RTAI_SITE)/$(RTAI_PATCH), i.e:

   https://www.rtai.org/userfiles/downloads/RTAI/home/foo/bar/myrtai.patch

Pretty clear that it has no chance of working.

Now, let's assume an URL is used as the value of
BR2_LINUX_KERNEL_EXT_RTAI_PATCH, such as
http://foo.com/bar/myrtai.patch. In this case, it will be properly
downloaded by the package infrastructure. But then, the following code
kicks in:

define RTAI_PREPARE_KERNEL
       $(APPLY_PATCHES)                        \
               $(LINUX_DIR)                    \
               $(dir $(RTAI_PATCH))            \
               $(notdir $(RTAI_PATCH))
endef

The value of $(dir $(RTAI_PATCH)) will be http://foo.com/bar/. How
can $(APPLY_PATCHES) make use of such a stupid patch location?

[Thomas: add Config.in.legacy handling, as suggested by Arnout, even
if we believe that no-one could have ever used this option.]

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Acked-by: "Yann E. MORIN" <yann.morin.1998@free.fr>