external/mbedtls: fix config certificate mpi size
Basically mbedtls The readable certificate supports an mpi size of 256 bytes. However, a connection to the FOTA server requires a 512 byte certificate in mpi size. Therefore, modify the mpi size option to 512.
/* MPI / BIGNUM options */
//#define MBEDTLS_MPI_WINDOW_SIZE 6 /**< Maximum windows size used. */
-#define MBEDTLS_MPI_MAX_SIZE 256 /**< Maximum number of bytes for usable MPIs. */
+#define MBEDTLS_MPI_MAX_SIZE 512 /**< Maximum number of bytes for usable MPIs. */
/* CTR_DRBG options */
//#define MBEDTLS_CTR_DRBG_ENTROPY_LEN 48 /**< Amount of entropy used per seed by default (48 with SHA-512, 32 with SHA-256) */
