ANDROID: db845c: Enable device tree overlay support

This DTC_FLAGS will build the base platform dtbs with
overlay support. Otherwise fdtoverlay command throws
"FDT_ERR_NOTFOUND" error at the build time.

Missing/broken DTBO partition support on DB845c and RB5 is
the primary reason we have to apply DTB overlays at the
build time.

This patch will help enable currently out-of-tree/WIP (GKI)
features on supported devboards, and help us start testing
them in the LKFT builds.

Bug: 146449535
Signed-off-by: Amit Pundir <amit.pundir@linaro.org>
Change-Id: Ie1510d93532544683a1408e88473da5a83fc3a60