Bump imgtool version to 1.6.0-rc1.
Bump imgtool version to 1.6.0-rc1.

Signed-off-by: David Brown <0af22db655f3ade5845314d695c1f4910314b0cb@linaro.org>

utzig/mcuboot,utzig/mcuboot,utzig/mcuboot,runtimeco/mcuboot,ATmobica/mcuboot,utzig/mcuboot,tamban01/mcuboot,runtimeco/mcuboot,tamban01/mcuboot,runtimeco/mcuboot,ATmobica/mcuboot,utzig/mcuboot,tamban01/mcuboot,ATmobica/mcuboot,ATmobica/mcuboot,ATmobica/mcuboot,runtimeco/mcuboot,tamban01/mcuboot,tamban01/mcuboot,runtimeco/mcuboot