powerpc/tool/pbl: fix pbl image compiling process

Previous process of compiling a PBL boot image is:
1: make <board_name_config>
2: make u-boot.pbl

for example:
make T4240QDS_SDCARD_config
make u-boot.pbl

Now the process is:
1: make <board_name>

for example:
make T4240QDS_SDCARD

Also, updated README.pblimage.

Signed-off-by: Shaohui Xie <Shaohui.Xie@freescale.com>