Add SPDX license identifiers

Add license information to files. Mostly automated with:

    find src/ -name '*.[c,h]' | xargs sed -i '1s,^,// SPDX-License-Identifier: GPL-3.0-only\n\n,'
    find src/ -name '*.mk' | xargs sed -i '1s,^,# SPDX-License-Identifier: GPL-3.0-only\n\n,'

src/arch/avr/arch.mk

@@ -1,3 +1,5 @@
+# SPDX-License-Identifier: GPL-3.0-only
+
 CC=avr-gcc -mmcu=$(EC)
 CFLAGS+=-Os -fstack-usage -Wall -Werror -Wl,--gc-sections -Wl,-u,vfprintf -lprintf_flt
 OBJ=$(patsubst src/%.c,$(BUILD)/%.o,$(SRC))

src/arch/avr/gpio.c

@@ -1,3 +1,5 @@
+// SPDX-License-Identifier: GPL-3.0-only
+
 #include <arch/gpio.h>
 
 bool gpio_get_dir(struct Gpio * gpio) {

src/arch/avr/i2c.c

@@ -1,3 +1,5 @@
+// SPDX-License-Identifier: GPL-3.0-only
+
 #include <stdio.h>
 #include <avr/io.h>
 #include <util/twi.h>

src/arch/avr/i2c_slave.c

@@ -1,3 +1,5 @@
+// SPDX-License-Identifier: GPL-3.0-only
+
 // Based on https://github.com/thegouger/avr-i2c-slave
 
 #include <stdio.h>

src/arch/avr/include/arch/gpio.h

@@ -1,3 +1,5 @@
+// SPDX-License-Identifier: GPL-3.0-only
+
 #ifndef _ARCH_GPIO_H
 #define _ARCH_GPIO_H
 

src/arch/avr/include/arch/i2c_slave.h

@@ -1,3 +1,5 @@
+// SPDX-License-Identifier: GPL-3.0-only
+
 #ifndef _ARCH_I2C_SLAVE_H
 #define _ARCH_I2C_SLAVE_H
 

src/arch/avr/include/arch/uart.h

@@ -1,3 +1,5 @@
+// SPDX-License-Identifier: GPL-3.0-only
+
 #ifndef _ARCH_UART_H
 #define _ARCH_UART_H
 

src/arch/avr/uart.c

@@ -1,3 +1,5 @@
+// SPDX-License-Identifier: GPL-3.0-only
+
 #include <stdio.h>
 #include <avr/io.h>