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/M5249EVB_defconfig b/configs/M5249EVB_defconfig
index 9a360b0..65606c4 100644
--- a/configs/M5249EVB_defconfig
+++ b/configs/M5249EVB_defconfig
@@ -14,6 +14,7 @@
 CONFIG_SYS_DEVICE_NULLDEV=y
 # CONFIG_DISPLAY_BOARDINFO is not set
 # CONFIG_CMDLINE_EDITING is not set
+CONFIG_SYS_PBSIZE=276
 CONFIG_CMD_IMLS=y
 CONFIG_LOOPW=y
 CONFIG_CMD_MX_CYCLIC=y