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/M5272C3_defconfig b/configs/M5272C3_defconfig
index ed48264..59c4922 100644
--- a/configs/M5272C3_defconfig
+++ b/configs/M5272C3_defconfig
@@ -14,6 +14,7 @@
 # CONFIG_CMDLINE_EDITING is not set
 # CONFIG_AUTO_COMPLETE is not set
 CONFIG_SYS_PROMPT="-> "
+CONFIG_SYS_PBSIZE=276
 CONFIG_CMD_IMLS=y
 # CONFIG_CMD_LOADB is not set
 # CONFIG_CMD_LOADS is not set