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/P5040DS_SPIFLASH_defconfig b/configs/P5040DS_SPIFLASH_defconfig
index 32fec67..1e1a2f8 100644
--- a/configs/P5040DS_SPIFLASH_defconfig
+++ b/configs/P5040DS_SPIFLASH_defconfig
@@ -26,6 +26,7 @@
 CONFIG_BOARD_EARLY_INIT_R=y
 CONFIG_ID_EEPROM=y
 CONFIG_HUSH_PARSER=y
+CONFIG_SYS_PBSIZE=276
 CONFIG_CMD_IMLS=y
 CONFIG_CMD_GREPENV=y
 CONFIG_CMD_I2C=y