defconfig: msm8937_64: 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: I487e43861d4dcde0b0c35213a254af46d1e9aa50
Signed-off-by: Rahul Shahare <rshaha@codeaurora.org>