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/bootloader.ld

@@ -1,6 +1,6 @@
 MEMORY
 {
-    ICTM (rwx) : ORIGIN = 0x00000000, LENGTH = 16384
+    ICTM (rwx) : ORIGIN = 0x00000000, LENGTH = 2048
 }
 
 SECTIONS
@@ -30,5 +30,5 @@ SECTIONS
         __exidx_end = .;
     } > ICTM
 
-    _initial_stack_pointer = 16384;
+    _initial_stack_pointer = 2048;
 }