The C runtime wasn't initialized correctly (there was garbage in the data
and bss sections). Add a reset_handler which initializes these sections
before calling the application's main() function.
The initial stack pointer is also defined in the linker script, allowing the
application to override with a linker command line option
"-Wl,--defsym,_stack=0x20005000".
Thanks to Gareth McMullin <gareth@blacksphere.co.nz>.
