Update to 1.9.0-rc2

Bump the version number in the readme, and imgtool for the 1.9.0-rc2
release.
Signed-off-by: David Brown <david.brown@linaro.org>

README.md

@@ -16,7 +16,7 @@
 [travis]: https://travis-ci.org/mcu-tools/mcuboot
 [license]: https://github.com/mcu-tools/mcuboot/blob/main/LICENSE
 
-This is MCUboot version 1.9.0-rc1
+This is MCUboot version 1.9.0-rc2
 
 MCUboot is a secure bootloader for 32-bits microcontrollers. It defines a
 common infrastructure for the bootloader and the system flash layout on

scripts/imgtool/__init__.py

@@ -14,4 +14,4 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
-imgtool_version = "1.9.0rc1"
+imgtool_version = "1.9.0rc2"