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/bcm963158_ram_defconfig b/configs/bcm963158_ram_defconfig
index cd622d6..944f8be 100644
--- a/configs/bcm963158_ram_defconfig
+++ b/configs/bcm963158_ram_defconfig
@@ -21,6 +21,7 @@
 CONFIG_DISPLAY_BOARDINFO_LATE=y
 CONFIG_HUSH_PARSER=y
 CONFIG_SYS_MAXARGS=24
+CONFIG_SYS_PBSIZE=276
 # CONFIG_CMD_LZMADEC is not set
 # CONFIG_CMD_UNZIP is not set
 # CONFIG_CMD_FLASH is not set