Convert CONFIG_SYS_PBSIZE to Kconfig

This converts the following to Kconfig:
   CONFIG_SYS_PBSIZE

Signed-off-by: Tom Rini <trini@konsulko.com>
diff --git a/configs/smdk5420_defconfig b/configs/smdk5420_defconfig
index 7f2e42a..1658eea 100644
--- a/configs/smdk5420_defconfig
+++ b/configs/smdk5420_defconfig
@@ -24,6 +24,7 @@
 CONFIG_CONSOLE_MUX=y
 # CONFIG_SPL_FRAMEWORK is not set
 CONFIG_SYS_PROMPT="SMDK5420 # "
+CONFIG_SYS_PBSIZE=1024
 CONFIG_CMD_GPIO=y
 CONFIG_CMD_I2C=y
 CONFIG_CMD_MMC=y