sbc8349: combine HRCW flash and u-boot image flash

Up to this point in time, the sbc8349 board was storing the u-boot
image in flash 2x.  One for the HRCW value at the beginning of
flash (0xff80_0000), and once close to the end of flash (0xfff8_0000)
for the actual image that got executed.

This moves the TEXT_BASE to be the beginning of flash, which makes
the second copy of the image redundant, and frees up the flash
from the end of the environment storage to the end of the flash
device itself.

Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>
Signed-off-by: Kim Phillips <kim.phillips@freescale.com>