linux-dtb.inc: Replace /boot/ with /${KERNEL_IMAGEDEST}/

linux-dtb.inc: Replace /boot/ with /${KERNEL_IMAGEDEST}/

Devicetree files were installed hard-coded in /boot. When KERNEL_IMAGEDEST
is anything else but "boot", the postinstall script and the file locations
no longer match and the postinstall will fail.

Replace "boot" with "${KERNEL_IMAGEDEST}" to fix this problem, and to allow
the devicetree files to be installed in another location.

(From OE-Core rev: 0f589b9a38397fdf55025062a45889b19d1c83c4)

Signed-off-by: Mike Looijmans <mike.looijmans@topic.nl>
Signed-off-by: Saul Wold <sgw@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>