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/nitrogen6q_defconfig b/configs/nitrogen6q_defconfig
index 0483417..e169350 100644
--- a/configs/nitrogen6q_defconfig
+++ b/configs/nitrogen6q_defconfig
@@ -30,6 +30,7 @@
 CONFIG_MISC_INIT_R=y
 CONFIG_HUSH_PARSER=y
 CONFIG_SYS_MAXARGS=32
+CONFIG_SYS_PBSIZE=532
 CONFIG_CMD_BOOTZ=y
 CONFIG_CMD_MEMTEST=y
 CONFIG_SYS_ALT_MEMTEST=y