Now that we have a better understanding of what's causing the issue
that prevented entering FEL sometimes, we can adjust the workaround
code to a proper solution, i.e. skip over the problematic location.

Since the code amounts to less than a dozen ARM instructions, I've
decided to rewrite it as assembly code - fel-sdboot.S replaces the
former fel-sdboot.c.

The commit also includes a new binary (bin/fel-sdboot.sunxi) with
these changes.
Signed-off-by: Bernhard Nortmann <bernhard.nortmann@web.de>
Reviewed-by: Siarhei Siamashka <siarhei.siamashka@gmail.com>