ARM: boot: Fix usage of kecho

Since commit edc88ceb0 (ARM: be really quiet when building with 'make -s') the
following output is generated when building a kernel for ARM:

echo '  Kernel: arch/arm/boot/Image is ready'
  Kernel: arch/arm/boot/Image is ready
  Building modules, stage 2.
echo '  Kernel: arch/arm/boot/zImage is ready'
  Kernel: arch/arm/boot/zImage is ready

As per Documentation/kbuild/makefiles.txt the correct way of using kecho is
'@$(kecho)'.

Make this change so no more unwanted 'echo' messages are displayed.

Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>