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/mk808_defconfig b/configs/mk808_defconfig
index 20bca75..e94835b 100644
--- a/configs/mk808_defconfig
+++ b/configs/mk808_defconfig
@@ -38,6 +38,7 @@
 CONFIG_SPL_FS_EXT4=y
 CONFIG_SYS_MMCSD_FS_BOOT_PARTITION=2
 CONFIG_TPL_NEEDS_SEPARATE_STACK=y
+CONFIG_SYS_PBSIZE=276
 # CONFIG_BOOTM_PLAN9 is not set
 # CONFIG_BOOTM_RTEMS is not set
 # CONFIG_BOOTM_VXWORKS is not set