mbedtls_gcm_starts_soft() function

When the MBEDTLS_GCM_NON_AES_CIPHER_SOFT_FALLBACK is defined, for non-AES GCM operations we need to fallback to the software function definitions of the mbedtls GCM layer. Thus in this case we need declarations for the software funtions. Please refer mbedtls/include/mbedtls/gcm.h for function documentations

mbedtls_gcm_context::buf contains the partial state of the computation of the authentication tag. mbedtls_gcm_context::add_len and mbedtls_gcm_context::len indicate different stages of the computation: * len == 0 && add_len == 0: initial state * len == 0 && add_len % 16 != 0: the first `add_len % 16` bytes have a partial block of AD that has been xored in but not yet multiplied in. * len == 0 && add_len % 16 == 0: the authentication tag is correct if the data ends now. * len % 16 != 0: the first `len % 16` bytes have a partial block of ciphertext that has been xored in but not yet multiplied in. * len > 0 && len % 16 == 0: the authentication tag is correct if the data ends now.