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/espresso7420_defconfig b/configs/espresso7420_defconfig
index 533757b..e06eb99 100644
--- a/configs/espresso7420_defconfig
+++ b/configs/espresso7420_defconfig
@@ -17,6 +17,7 @@
 # CONFIG_AUTO_COMPLETE is not set
 # CONFIG_SYS_LONGHELP is not set
 CONFIG_SYS_PROMPT="ESPRESSO7420 # "
+CONFIG_SYS_PBSIZE=1024
 # CONFIG_CMD_SETEXPR is not set
 CONFIG_SYS_RELOC_GD_ENV_ADDR=y
 # CONFIG_MMC is not set