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/silk_defconfig b/configs/silk_defconfig
index 9ae358d..e77578f 100644
--- a/configs/silk_defconfig
+++ b/configs/silk_defconfig
@@ -36,6 +36,7 @@
 CONFIG_SYS_SPI_U_BOOT_OFFS=0x140000
 CONFIG_SPL_YMODEM_SUPPORT=y
 CONFIG_HUSH_PARSER=y
+CONFIG_SYS_PBSIZE=256
 CONFIG_CMD_BOOTZ=y
 # CONFIG_CMD_IMI is not set
 # CONFIG_CMD_XIMG is not set