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/bk4r1_defconfig b/configs/bk4r1_defconfig
index 324a80e..7fc3c27 100644
--- a/configs/bk4r1_defconfig
+++ b/configs/bk4r1_defconfig
@@ -30,6 +30,7 @@
 CONFIG_USE_PREBOOT=y
 CONFIG_BOARD_EARLY_INIT_F=y
 CONFIG_HUSH_PARSER=y
+CONFIG_SYS_PBSIZE=276
 # CONFIG_CMD_ELF is not set
 CONFIG_CMD_MEMTEST=y
 CONFIG_CMD_DM=y