linux: make BR2_LINUX_KERNEL_DTB_IS_SELF_BUILT public

linux: make BR2_LINUX_KERNEL_DTB_IS_SELF_BUILT public

Currently the BR2_LINUX_KERNEL_DTB_IS_SELF_BUILT option is only
available to be selected by another config option. This option controls
if the Linux build system should build the DTB itself, or if buildroot
explicitly calls the Linux Makefile like "make something.dtb".

My use case: I want to build an OpenRISC image with a custom device tree
file. OpenRISC does not support appended DTBs, and I'm not using a
bootloader, but link everything into a single kernel (vmlinux) image.

The kernel option CONFIG_OPENRISC_BUILTIN_DTB allows me to specify a dtb
file, which is typically located in the kernel source tree in
arch/openrisc/boot/dts/NAME.dts.

Since my dts file is not part of the upstream kernel, I have two
options: either patch the kernel to include the DTS file, and then
specify it using the CONFIG_OPENRISC_BUILTIN_DTB option.

Or use buildroot's BR2_LINUX_KERNEL_CUSTOM_DTS_PATH config option to
copy any DTS file to the kernel source tree, and then specify this file
with CONFIG_OPENRISC_BUILTIN_DTB as before.

However, the second option also requries the buildroot option
BR2_LINUX_KERNEL_DTB_IS_SELF_BUILT to be set, otherwise I end up with a
"make -C kerneldir NAME.dts" call, which doesn't work (no such target
exists).

Currently the BR2_LINUX_KERNEL_DTB_IS_SELF_BUILT option exists, but it's
not visible/available. Simply making it available solves the problem for
me nicely.

Signed-off-by: Philipp Wagner <mail@philipp-wagner.com>
[Arnout: add help text]
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>