Rename ext/mbedtls to ext/mbedtls-asn1

The ext/mbedtls directory has a copy of the ASN.1 parser from the mbed
TLS library.  To allow a future change to bring the entire mbed TLS
library in as a submodule under ext, rename this to mbedtls-asn1 to make
it clear this is only a subset.

Signed-off-by: David Brown <david.brown@linaro.org>
diff --git a/boot/zephyr/CMakeLists.txt b/boot/zephyr/CMakeLists.txt
index ded117d..f5e02c7 100644
--- a/boot/zephyr/CMakeLists.txt
+++ b/boot/zephyr/CMakeLists.txt
@@ -66,7 +66,7 @@
 set(FIAT_DIR "${MCUBOOT_DIR}/ext/fiat")
 assert_exists(FIAT_DIR)
 # Path to mbed-tls' asn1 parser library.
-set(MBEDTLS_ASN1_DIR "${MCUBOOT_DIR}/ext/mbedtls")
+set(MBEDTLS_ASN1_DIR "${MCUBOOT_DIR}/ext/mbedtls-asn1")
 assert_exists(MBEDTLS_ASN1_DIR)
 set(NRF_DIR "${MCUBOOT_DIR}/ext/nrf")