sparc: Use $(kecho) to announce kernel images being ready

sparc: Use $(kecho) to announce kernel images being ready

My build test setup compiles allmodconfig all various architectures
(arm64 m68k powerpc riscv s390 sparc64 x86_64) using make -s. If there
is no warning, the only output is

          kernel: arch/sparc/boot/image is ready
          kernel: arch/sparc/boot/zImage is ready

from the sparc64 build. Copy the incantation from x86 which is silent
when building with make -s and also mentions a version indication.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Acked-by: Sam Ravnborg <sam@ravnborg.org>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>