defconfig: sm8150: Disable BUILD_ARM64_APPENDED_DTB_IMAGE flag

Add support to compile the kernel without the dtbs appended to it.
All the dtbs are concatenated to form dtb.img. Boot image with header
version 2 will now have this dtb.img added towards the end at an offset.

Change-Id: I5340f657ab23ef6af3177ef7ac58f3f567b79abc
Signed-off-by: Prakruthi Deepak Heragu <pheragu@codeaurora.org>