ARM: be really quiet when building with 'make -s'

Sometimes we want the kernel build process to only print messages
on errors, e.g. in automated build testing. This uses the "kecho"
macro that the build system provides to hide a few informational
messages. Nothing changes for a regular "make" or "make V=1".

Without this patch, building any ARM kernel results in:

  Kernel: arch/arm/boot/Image is ready
  Kernel: arch/arm/boot/zImage is ready

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Nicolas Pitre <nicolas.pitre@linaro.org>
Cc: Russell King <rmk+kernel@arm.linux.org.uk>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Michal Marek <mmarek@suse.cz>