Determine MBEDTLS_ECP_MAX_BITS automatically

MBEDTLS_ECP_MAX_BITS doesn't make sense as a configuration option: it
must not be smaller than the largest supported curve, and it's useless
to set it to a larger value. So unconditionally set it to the size of
the largest supported curve. Remove it from the build configuration.

Alternative implementations must no longer need define this macro.

Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
diff --git a/configs/config-thread.h b/configs/config-thread.h
index bce9668..c1937de 100644
--- a/configs/config-thread.h
+++ b/configs/config-thread.h
@@ -85,8 +85,7 @@
 #define MBEDTLS_AES_ROM_TABLES
 
 /* Save RAM by adjusting to our exact needs */
-#define MBEDTLS_ECP_MAX_BITS             256
-#define MBEDTLS_MPI_MAX_SIZE              32 // 256 bits is 32 bytes
+#define MBEDTLS_MPI_MAX_SIZE              32 // 256-bit EC curve = 32 bytes
 
 /* Save ROM and a few bytes of RAM by specifying our own ciphersuite list */
 #define MBEDTLS_SSL_CIPHERSUITES MBEDTLS_TLS_ECJPAKE_WITH_AES_128_CCM_8