wimax/i2400m: support extended firmware format

The SBCF firmware format has been extended to support extra headers
after the main payload. These extra headers are used to sign the
firmware code with more than one certificate. This eases up
distributing single code images that work in more than one SKU of the
device.

The changes to support this feature will be spread in a series of
commits. This one just adds the support to parse the extra headers and
store them in i2400m->fw_hdrs. Coming changes to the loader code will
use that to determine which header to upload to the device.

The i2400m_fw_check() function now iterates over all the headers and
for each, calls i2400m_fw_hdr_check(), which does some basic checks on
each header. It then stores the headers for the bootloader code to use.

The i2400m_dev_bootstrap() function has been modified to cleanup
i2400m->fw_hdrs when done.

Signed-off-by: Inaky Perez-Gonzalez <inaky@linux.intel.com>