This shrinks the resulting binary size by ~25%. Exceptions
are still handled in arm mode, so we have to explicitely
put .arm directives into the exception code. Thumb-2 mode
has been tested on i.MX51 Babbage board.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>

Traditionally U-Boot and barebox have the exception vectors at
the start of the binary. There is no real reason in doing so,
because in the majority of cases this data will not be at 0x0
where it could be used as vectors directly anyway.
This patch puts the vectors into a separate linker section and
defines an head function which is placed at the start of the
image instead. Putting this in a separate function also has
the advantage that it can be placed at the start of images
which require an additional header like several Freescale i.MX
images. As the head function contains the barebox arm magic
those images can now also be detected as barebox images.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>