Changes
   * Create 4 byte reading macros in library/common.h, used in files
     within the same directory: MBEDTLS_BYTE_0... MBEDTLS_BYTE_3.
   * Move the (PUT and GET) UINT32_ (BE and LE) macro functions into
     library/common.h. Rename with the prefix MBEDTLS_ to satisfy
     test/scripts/check-names.sh (e.g MBEDTLS_PUT_UINT32_LE).
   * Move BYTES_TO_U32_LE macro function to library/common.h, also given
     the prefix MBEDTLS_.
     Fixes #4274.