linux: introduce BR2_KERNEL_DTB_KEEP_DIRNAME

linux: introduce BR2_KERNEL_DTB_KEEP_DIRNAME

In linux, newer architectures like ARM64 and RISC-V keep their device
tree sources organized by subdirectories. When these device trees are
installed by the kernel they will keep the directory names. But
buildroot strips the prefixes when installing them into /boot or the
images directory.
Sometimes the bootloader references the device tree by name (e.g.
u-boot has the environment variable 'fdtfile') which also includes
the prefix directory.
Make it possible to keep this prefix during installation, so we can
be compatible with other distributions and make it easier for the
user, because he doesn't have to change that environment variable.

Signed-off-by: Michael Walle <michael@walle.cc>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>