Implement SHA512_NO_SHA384 in sha512 module

Saves 140 bytes on sha512.o, measured with:

arm-none-eabi-gcc -Wall -Wextra -Iinclude -Os -mcpu=cortex-m0plus -mthumb -c library/sha512.c && arm-none-eabi-size sha512.o

arm-none-eabi-gcc (GNU Tools for Arm Embedded Processors 7-2018-q2-update) 7.3.1 20180622 (release) [ARM/embedded-7-branch revision 261907]

Todo:
- fix selftest
- fix dependencies in test suites
- implement in MD layer

include/mbedtls/sha512.h

@@ -59,8 +59,10 @@ typedef struct mbedtls_sha512_context
     uint64_t total[2];          /*!< The number of Bytes processed. */
     uint64_t state[8];          /*!< The intermediate digest state. */
     unsigned char buffer[128];  /*!< The data block being processed. */
+#if !defined(MBEDTLS_SHA512_NO_SHA384)
     int is384;                  /*!< Determines which function to use:
                                      0: Use SHA-512, or 1: Use SHA-384. */
+#endif
 }
 mbedtls_sha512_context;