ARM: boot: Silence 'zimage-dtb is ready' message

We should use kecho here instead of echo, so that make -s will
skip printing anything here. Otherwise, builds with make -s will
be confused and consider this informational message a
warning/error.

Change-Id: I4c854636e5b8b7e8b11eba8e5a52824ebee50ea1
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>