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/cubieboard7_defconfig b/configs/cubieboard7_defconfig
index 35fd580..1a930ca 100644
--- a/configs/cubieboard7_defconfig
+++ b/configs/cubieboard7_defconfig
@@ -14,6 +14,7 @@
 # CONFIG_DISPLAY_BOARDINFO is not set
 CONFIG_SYS_PROMPT="U-Boot => "
 CONFIG_SYS_MAXARGS=64
+CONFIG_SYS_PBSIZE=1051
 CONFIG_CMD_MMC=y
 CONFIG_MMC_OWL=y
 CONFIG_PHY_REALTEK=y