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/8051/arch.c

@@ -1,3 +1,5 @@
+// SPDX-License-Identifier: GPL-3.0-only
+
 #include <8051.h>
 
 #include <arch/arch.h>

src/arch/8051/arch.mk

@@ -1,3 +1,5 @@
+# SPDX-License-Identifier: GPL-3.0-only
+
 CC=sdcc -mmcs51 --model-large --Werror
 OBJ=$(patsubst src/%.c,$(BUILD)/%.rel,$(SRC))
 

src/arch/8051/delay.c

@@ -1,3 +1,5 @@
+// SPDX-License-Identifier: GPL-3.0-only
+
 // Uses timer 1 to implement delays
 
 #include <8051.h>

src/arch/8051/include/arch/arch.h

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

src/arch/8051/include/arch/delay.h

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

src/arch/8051/include/arch/time.h

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

src/arch/8051/time.c

@@ -1,3 +1,5 @@
+// SPDX-License-Identifier: GPL-3.0-only
+
 // Uses timer 0 to keep track of global time
 
 #include <8051.h>