powerpc:Rename CONFIG_PBLRCW_CONFIG & CONFIG_SYS_FSL_PBL_PBI

Rename CONFIG_PBLRCW_CONFIG and CONFIG_PBLRCW_CONFIG.

Also add their details in README.

Signed-off-by: Prabhakar Kushwaha <prabhakar@freescale.com>
Reviewed-by: York Sun <yorksun@freescale.com>
diff --git a/Makefile b/Makefile
index 47a03e3..1687e2e 100644
--- a/Makefile
+++ b/Makefile
@@ -419,8 +419,8 @@
 		-a $(CONFIG_SYS_TEXT_BASE) -e $(CONFIG_SYS_TEXT_BASE) -d $< $@
 
 $(obj)u-boot.pbl:	$(obj)u-boot.bin
-		$(obj)tools/mkimage -n $(CONFIG_PBLRCW_CONFIG) \
-		-R $(CONFIG_PBLPBI_CONFIG) -T pblimage \
+		$(obj)tools/mkimage -n $(CONFIG_SYS_FSL_PBL_RCW) \
+		-R $(CONFIG_SYS_FSL_PBL_PBI) -T pblimage \
 		-d $< $@
 
 $(obj)u-boot.sha1:	$(obj)u-boot.bin