zephyr: migrate signature type to Kconfig

Handle the CONFIG_BOOT_SIGNATURE_TYPE_xxx values in Zephyr's
mcuboot_config.h by converting them into the platform-agnostic MCUboot
definitions.

This requires some changes to the way the release test Makefile is
structured, since Kconfig symbols cannot be set from the command line.

Instead, use the OVERLAY_CONFIG feature of the Zephyr build system,
which allows specifying extra fragments to merge into the final
.config. (This is an orthogonal mechanism to setting CONF_FILE; it is
used by Zephyr's CI script sanitycheck to add additional fragments, so
it's appropriate for use by MCUboot's testing scripts as well.)

We additionally need to move to a single prj.conf file due to a
dependency issue. We can no longer determine CONF_FILE from the
signature type, since that is now determined from the final .config or
autoconf.h, which is a build output that depends on CONF_FILE.

To move to a single prj.conf:

- delete prj-p256.conf and adjust prj.conf to serve both signature types
- add a top-level mbedTLS configuration file which dispatches to
  the right sub-header depending on the key type
- as a side effect, have the simulator pick the right config file
  depending on the case

This fixes and cleans up quite a bit of the signature type handling,
which had become something of a mess over time. For example, it fixes
a bug in ECDSA mode's configuration that wasn't actually selecting
config-asn1.h, and forces the simulator to use the same mbedTLS
configuration file as builds for real hardware.

Finally, we also have to move the mbedTLS vs. TinyCrypt choice into
mcuboot_config.h at the same time as well, since CMakeLists.txt was
making that decision based on the signature type.

Signed-off-by: Marti Bolivar <marti@opensourcefoundries.com>
diff --git a/boot/zephyr/os.c b/boot/zephyr/os.c
index deb9dce..dd0d7e4 100644
--- a/boot/zephyr/os.c
+++ b/boot/zephyr/os.c
@@ -22,7 +22,7 @@
 
 #include "os/os_heap.h"
 
-#ifdef MCUBOOT_USE_MBED_TLS
+#ifdef CONFIG_BOOT_USE_MBEDTLS
 
 #include <mbedtls/platform.h>
 #include <mbedtls/memory_buffer_alloc.h>