[util] Add a binary to hex string convertor

The Issuer and Subject are both tstr (text-string) type.
Using key_id as issuer/subject requires a conventor.

Bug: 356532759
Signed-off-by: Tommy Chiu <tommychiu@google.com>

sw/device/silicon_creator/lib/base/util.c

@@ -22,3 +22,16 @@ void util_reverse_bytes(void *buf, size_t num_bytes) {
     byte_buf[num_bytes - i - 1] = temp;
   }
 }
+
+// Returns hexdump character for the half-byte.
+static inline uint8_t hexdump_halfbyte(uint8_t half_byte) {
+  if (half_byte < 10)
+    return '0' + half_byte;
+  else
+    return 'a' + half_byte - 10;
+}
+
+void util_hexdump_byte(uint8_t byte, uint8_t *str) {
+  str[0] = hexdump_halfbyte((byte & 0xF0) >> 4);
+  str[1] = hexdump_halfbyte(byte & 0x0F);
+}

sw/device/silicon_creator/lib/base/util.h

@@ -37,6 +37,15 @@ uint32_t util_size_to_words(uint32_t bytes);
  */
 void util_reverse_bytes(void *buf, size_t num_bytes);
 
+/**
+ * Fills hexdump of the byte (lowercase).
+ *
+ * @param byte Byte to convert to a hex string.
+ * @param[out] str String buffer (always 2 bytes) to place hex string encoded
+ * byte in.
+ */
+void util_hexdump_byte(uint8_t byte, uint8_t *str);
+
 #ifdef __cplusplus
 }
 #endif