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/app.ld

@@ -0,0 +1,34 @@
+MEMORY
+{
+    ICTM (rwx) : ORIGIN = 0x00000800, LENGTH = 12228
+}
+
+SECTIONS
+{
+    .text :
+    {
+        KEEP(*(.app_init))
+
+        *(.text*)
+        *(.rodata*)
+    } > ICTM
+
+    .bss (NOLOAD) :
+    {
+        _bss_begin = .;
+        *(.bss*)
+        *(COMMON)
+        _bss_end = .;
+    } > ICTM
+
+    .data :
+    {
+        *(.data*)
+
+        __exidx_start = .;
+        *(.exidx*)
+        __exidx_end = .;
+    } > ICTM
+
+    _initial_stack_pointer = 16384;
+}