arm: working bootloader, example app

'make' will produce two outputs:
- bootloader.elf to be loaded into the bitstream
- app.bin to be loaded through the programmer

Loading app.bin is as simple as:
- reset the board
- `python3 prog.py app.bin`

arm/prog.py

@@ -0,0 +1,47 @@
+import serial
+import struct
+import sys
+
+offset = 0x800
+tty = 'tty'
+baud = 115200
+chunksize = 128
+
+
+def write(s, offset, dat):
+    for i in range(0, len(dat), chunksize):
+        chunk = dat[i: i + chunksize]
+        cmd = struct.pack('<cII', b'c', offset + i, len(chunk))
+
+        print(f'Sending {len(chunk)} bytes @0x{offset + i:04x}')
+
+        s.write(cmd)
+        s.write(chunk)
+
+        ack = s.read(2)
+        if len(ack) < 2:
+            raise RuntimeError(f'timeout waiting for full ack. got {ack}')
+        if ack[0] != b'a'[0]:
+            raise RuntimeError(f'expected ack, got this instead: {ack}')
+        print(f'Ack! len={ack[1]}')
+
+
+def jump(s, offset):
+        addr = struct.pack('<I', offset)
+
+        s.write(b'j')
+        s.write(addr)
+
+
+def main():
+    binfile = sys.argv[1]
+    with open(binfile, 'rb') as f:
+        dat = f.read()
+
+    s = serial.Serial(tty, baud, timeout=1)
+    write(s, offset, dat)
+    jump(s, offset)
+
+
+if __name__ == "__main__":
+    main()