add tx offset macro

Author: Tarak Ben Youssef

app/src/common/tx.c

@@ -44,15 +44,17 @@ storage_t NV_CONST N_appdata_impl __attribute__ ((aligned(64)));
 
 parser_context_t ctx_parsed_tx;
 
-#define DOMAIN_TAG_LENGTH 32
 // UTF-8 encoding of "FLOW-V0.0-transaction" padded with zeros to 32 bytes
+#define DOMAIN_TAG_LENGTH 32
 const uint8_t TX_DOMAIN_TAG[DOMAIN_TAG_LENGTH] = {\
     0x46, 0x4C, 0x4F, 0x57, 0x2D, 0x56, 0x30, 0x2E, 
     0x30, 0x2D, 0x74, 0x72, 0x61, 0x6E, 0x73, 0x61, 
     0x63, 0x74, 0x69, 0x6F, 0x6E,    0,    0,    0,
        0,    0,    0,    0,    0,    0,    0,    0,
 };
 
+#define TX_BUFFER_OFFSET DOMAIN_TAG_LENGTH
+
 void tx_initialize() {
     buffering_init(
             ram_buffer,
@@ -72,8 +74,8 @@ uint32_t tx_append(unsigned char *buffer, uint32_t length) {
 }
 
 uint32_t tx_get_buffer_length() {
-    if (buffering_get_buffer()->pos >= DOMAIN_TAG_LENGTH) {
-        return buffering_get_buffer()->pos - DOMAIN_TAG_LENGTH;
+    if (buffering_get_buffer()->pos >= TX_BUFFER_OFFSET) {
+        return buffering_get_buffer()->pos - TX_BUFFER_OFFSET;
     }
     return 0;
 }
@@ -83,7 +85,7 @@ uint32_t get_signable_length() {
 }
 
 uint8_t *tx_get_buffer() {
-    return buffering_get_buffer()->data + DOMAIN_TAG_LENGTH;
+    return buffering_get_buffer()->data + TX_BUFFER_OFFSET;
 }
 
 uint8_t *get_signable() {