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/edison_defconfig b/configs/edison_defconfig
index 3f27a2f..af70b73 100644
--- a/configs/edison_defconfig
+++ b/configs/edison_defconfig
@@ -15,6 +15,7 @@
 CONFIG_LAST_STAGE_INIT=y
 CONFIG_HUSH_PARSER=y
 CONFIG_SYS_MAXARGS=128
+CONFIG_SYS_PBSIZE=2068
 CONFIG_CMD_CPU=y
 CONFIG_CMD_ASKENV=y
 CONFIG_CMD_GREPENV=y