Preps for 1.8.0-rc1

Update version fields for 1.8.0-rc1 release.

Signed-off-by: David Brown <david.brown@linaro.org>
diff --git a/README.md b/README.md
index 47e67a7..60d6594 100644
--- a/README.md
+++ b/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 1.8.0-dev
+This is mcuboot version 1.8.0-rc1
 
 MCUboot is a secure bootloader for 32-bit MCUs. The goal of MCUboot is to
 define a common infrastructure for the bootloader, system flash layout on