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/vinco_defconfig b/configs/vinco_defconfig
index 9df9da4..85e9f62 100644
--- a/configs/vinco_defconfig
+++ b/configs/vinco_defconfig
@@ -35,6 +35,7 @@
 CONFIG_OF_CONTROL=y
 CONFIG_ENV_IS_IN_SPI_FLASH=y
 CONFIG_SYS_RELOC_GD_ENV_ADDR=y
+CONFIG_NET_RETRY_COUNT=20
 CONFIG_AT91_GPIO=y
 CONFIG_SUPPORT_EMMC_BOOT=y
 CONFIG_GENERIC_ATMEL_MCI=y