cmake: mcuboot: Add Kconfig for passing west sign arguments

The commit adds CONFIG_MCUBOOT_CMAKE_WEST_SIGN_PARAMS Kconfig
option to allow passing arguments to west sign.
The option is used to pass arguments that immediately follow
west sign in cmake invocations and replaces hardcoded
"--quiet" parameter, but defaults to the "--quiet" value to
keep current behaviour.
New Kconfig option allows to pass arguments to west sign
without modifying cmake/mcuboot.cmake.

Signed-off-by: Dominik Ermel <dominik.ermel@nordicsemi.no>