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/nsim_700_defconfig b/configs/nsim_700_defconfig
index 4dcb948..359edf1 100644
--- a/configs/nsim_700_defconfig
+++ b/configs/nsim_700_defconfig
@@ -14,6 +14,7 @@
 CONFIG_USE_BOOTARGS=y
 CONFIG_BOOTARGS="console=ttyS0,115200n8"
 CONFIG_SYS_PROMPT="nsim# "
+CONFIG_SYS_PBSIZE=279
 # CONFIG_CMD_SETEXPR is not set
 CONFIG_OF_CONTROL=y
 CONFIG_OF_EMBED=y