kernel/build: Append RTIC dtb in dtb image for boot header v2

With Boot image header version 2, dtb is no longer appended to
kernel image. dtb.img is generated separately. So concatenate RTIC
dtb to the dtb.img for the bootloader to read the rtic dtb and share
the measurement parameter to hypervisor. Make sure that the RTIC DTB
rule added regardless of the presence of rtic_mp.dts. Instead, check
this during the rule execution (never fail if RTIC DTS is missing).

Change-Id: Id2d86aa44ae04ef82d3808f40e5f1187a8ec9735
Signed-off-by: Amareswar Reddy Valasa <avalasa@codeaurora.org>