Convert CONFIG_NET_RETRY_COUNT to Kconfig

This converts the following to Kconfig:
   CONFIG_NET_RETRY_COUNT

Cc: Ramon Fried <rfried.dev@gmail.com>
Signed-off-by: Tom Rini <trini@konsulko.com>
diff --git a/configs/am335x_sl50_defconfig b/configs/am335x_sl50_defconfig
index 21c67bd..3fab49a 100644
--- a/configs/am335x_sl50_defconfig
+++ b/configs/am335x_sl50_defconfig
@@ -59,6 +59,7 @@
 CONFIG_SYS_MMC_ENV_PART=2
 CONFIG_ENV_VARS_UBOOT_RUNTIME_CONFIG=y
 CONFIG_VERSION_VARIABLE=y
+CONFIG_NET_RETRY_COUNT=10
 CONFIG_BOOTP_SEND_HOSTNAME=y
 CONFIG_BOOTCOUNT_LIMIT=y
 CONFIG_SYS_I2C_LEGACY=y