raspi3: fix the DEBUG_UART build and the documented key size

DEBUG_UART=1 did not compile: uart_init() declared an unused local and
-Werror rejected it, so the target's console output could never be
enabled. Drop the local.

The documented key file is 44 bytes, but AES256-CTR wants a 32-byte key
plus a 16-byte IV; sign fails with "Error reading IV" as written. Use
48 bytes, matching the key the HAL used to carry.

docs/Targets.md

@@ -3755,9 +3755,12 @@ make wolfboot.bin CROSS_COMPILE=aarch64-linux-gnu-
 * Create the decrypt key + nonce
 
 ```
-printf "0123456789abcdef0123456789abcdef0123456789ab" > /tmp/enc_key.der
+printf "0123456789abcdef0123456789abcdef0123456789abcdef" > /tmp/enc_key.der
 ```
 
+  AES256-CTR takes a 32-byte key followed by a 16-byte IV, so the file is
+  48 bytes. A shorter one fails with `Error reading IV`.
+
 * Provision the same key into the bootloader
 
   The raspi3 HAL no longer ships a hardcoded key, so the bootloader has

hal/raspi3.c

@@ -168,8 +168,6 @@ void uart_write(const char* buf, uint32_t sz) {
 
 void uart_init()
 {
-    register unsigned int c;
-
     /* initialize UART. turn off UART 0*/
     *UART0_CR = 0;