powerpc/CoreNet: Allow pbl images to take u-boot images != 512K

Instead of assuming that SYS_TEXT_BASE is 0xFFF80000 calculate the initial
pbl command offset by subtracting the image size from the top of the
24-bit address range. Also increase the size of the memory buffer to
accommodate a larger output image.

Signed-off-by: Chris Packham <chris.packham@alliedtelesis.co.nz>
Signed-off-by: Andy Fleming <afleming@freescale.com>