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/nyan-big_defconfig b/configs/nyan-big_defconfig
index 0c906fc..531cc02 100644
--- a/configs/nyan-big_defconfig
+++ b/configs/nyan-big_defconfig
@@ -25,6 +25,7 @@
 CONFIG_DISPLAY_BOARDINFO_LATE=y
 CONFIG_SYS_PROMPT="Tegra124 (Nyan-big) # "
 CONFIG_SYS_MAXARGS=64
+CONFIG_SYS_PBSIZE=2087
 # CONFIG_CMD_IMI is not set
 CONFIG_CMD_DFU=y
 CONFIG_CMD_GPIO=y