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/pico-nymph-imx7d_defconfig b/configs/pico-nymph-imx7d_defconfig
index 92c85ca..9d6a916 100644
--- a/configs/pico-nymph-imx7d_defconfig
+++ b/configs/pico-nymph-imx7d_defconfig
@@ -30,6 +30,7 @@
 CONFIG_SPL_USB_GADGET=y
 CONFIG_SPL_USB_SDP_SUPPORT=y
 CONFIG_SYS_MAXARGS=32
+CONFIG_SYS_PBSIZE=532
 # CONFIG_CMD_BOOTD is not set
 CONFIG_CMD_BOOTMENU=y
 # CONFIG_CMD_IMI is not set