image: Add boot_get_fpga() to load fpga with bootm

Add function boot_get_fpga() which find and load bitstream to
programmable logic if fpga entry is present.
Function is supported on Xilinx devices for full and partial bitstreams
in BIN and BIT format.

Signed-off-by: Michal Simek <michal.simek@xilinx.com>
Remove additional blankline in image.h