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>
diff --git a/Makefile b/Makefile
index dc04179..2d18d27 100644
--- a/Makefile
+++ b/Makefile
@@ -398,6 +398,7 @@
 
 ALL-$(CONFIG_NAND_U_BOOT) += $(obj)u-boot-nand.bin
 ALL-$(CONFIG_ONENAND_U_BOOT) += $(obj)u-boot-onenand.bin
+ALL-$(CONFIG_RAMBOOT_PBL) += $(obj)u-boot.pbl
 ALL-$(CONFIG_SPL) += $(obj)spl/u-boot-spl.bin
 ALL-$(CONFIG_SPL_FRAMEWORK) += $(obj)u-boot.img
 ALL-$(CONFIG_TPL) += $(obj)tpl/u-boot-tpl.bin