crc16: banked wrapper for cross-bank callers

The lookup tables live in BANK1, so the routine switches the code bank to
reach them with movc and never switches it back. That is the contract the
__naked declaration already implied: same bank or common segment only.
The name now says it.

crc16_bank1 is the routine unchanged. crc16 becomes a __banked wrapper in
BANK1, so a caller from another bank has its own bank put back by
__sdcc_banked_ret. The two existing call sites take the direct path and
pay nothing.

rtlplayground.c

@@ -32,7 +32,7 @@ extern __xdata uint32_t flash_size;
 
 extern __xdata uint16_t crc_value;
 __xdata struct machine_runtime machine_detected;
-void crc16(__xdata uint8_t *v) __naked;
+void crc16_bank1(__xdata uint8_t *v) __naked;
 void flash_default_config(void);
 void early_boot_handle_button(void);
 
@@ -2095,7 +2095,7 @@ void check_and_flash_update_image(void)
 			flash_read_bulk(flash_buf);
 			bptr = flash_buf;
 			for (j = 0; j < FLASH_BUF_SIZE; j++) {
-				crc16(bptr++);
+				crc16_bank1(bptr++);
 			}
 			source += FLASH_BUF_SIZE;
 			if (i%16 == 0) write_char('.');