docs: Fold additional release notes

Add in a few additional release notes for the 2.1 release.

Signed-off-by: David Brown <david.brown@linaro.org>

docs/release-notes.d/flash-reset.md

@@ -1,2 +0,0 @@
-- Use general flash operations to determine the flash reset vector. This
-  improves support a bit for some configurations of external flash.

docs/release-notes.d/hkdf-mem-leak.md

@@ -1 +0,0 @@
-- fix a memory leak in the HKDF implementation.

docs/release-notes.d/zephyr-boot-banner.md

@@ -1,4 +0,0 @@
-- Zephyr: Added a MCUboot banner which displays the version of
-  MCUboot being used and the version of zephyr. This can be
-  disabled by setting ``CONFIG_MCUBOOT_BOOT_BANNER=n`` which
-  will revert back to the default zephyr boot banner.

docs/release-notes.md

@@ -49,6 +49,13 @@
   failure if the main thread priority is below 0 (cooperative
   thread), this would prevent USB CDC from working as the driver
   would not have been able to fire callbacks.
+- Use general flash operations to determine the flash reset vector. This
+  improves support a bit for some configurations of external flash.
+- fix a memory leak in the HKDF implementation.
+- Zephyr: Added a MCUboot banner which displays the version of
+  MCUboot being used and the version of zephyr. This can be
+  disabled by setting ``CONFIG_MCUBOOT_BOOT_BANNER=n`` which
+  will revert back to the default zephyr boot banner.
 
 ## Version 2.0.0