Add hh (byte) format for printf and scanf

[earlephilhower]

Fixes #4660

Add in support for byte-wide scanf and printf (%hh..) in printf and
scanf. Allows reading into a byte array one byte at a time.

Simple test:

void setup() {
  Serial.begin(115200);
  Serial.printf("%hhx\n", 123+256);
  char b[4];
  int i=999;
  int r= sscanf("1 2 3 4 5", "%hhd %hhd %hhd %hhd %d", b,b+1,b+2,b+3, &i);
  Serial.printf("r=%d  %d %d %d %d %d\n", r, b[0], b[1], b[2], b[3], i);
}

void loop() {
}

Gives expected output

7b
r=5  1 2 3 4 5