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/malta64_defconfig b/configs/malta64_defconfig
index 88d3cbe..165ec03 100644
--- a/configs/malta64_defconfig
+++ b/configs/malta64_defconfig
@@ -14,6 +14,7 @@
 CONFIG_MISC_INIT_R=y
 CONFIG_HUSH_PARSER=y
 CONFIG_SYS_PROMPT="malta # "
+CONFIG_SYS_PBSIZE=281
 CONFIG_CMD_IMLS=y
 CONFIG_CMD_IDE=y
 # CONFIG_CMD_LOADB is not set