tools: socfpga: Add socfpga preloader signing to mkimage

Like many platforms, the Altera socfpga platform requires that the
preloader be "signed" in a certain way or the built-in boot ROM will
not boot the code.

This change automatically creates an appropriately signed preloader
from an SPL image.

The signed image includes a CRC which must, of course, be generated
with a CRC generator that the SoCFPGA boot ROM agrees with otherwise
the boot ROM will reject the image.

Unfortunately the CRC used in this boot ROM is not the same as the
Adler CRC in lib/crc32.c. Indeed the Adler code is not technically a
CRC but is more correctly described as a checksum.

Thus, the appropriate CRC generator is added to lib/ as crc32_alt.c.

Signed-off-by: Charles Manning <cdhmanning@gmail.com>
Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Chin Liang See <clsee@altera.com>
Cc: Dinh Nguyen <dinguyen@altera.com>
Cc: Albert Aribaud <albert.u.boot@aribaud.net>
Cc: Tom Rini <trini@ti.com>
Cc: Wolfgang Denk <wd@denx.de>
Cc: Pavel Machek <pavel@denx.de>
Acked-by: Pavel Machek <pavel@denx.de>

V2: - Zap unused constant
    - Explicitly print an error message in case of error
    - Rework the hdr_checksum() function to take the *header directly
      instead of a plan buffer pointer