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/ethernut5_defconfig b/configs/ethernut5_defconfig
index 1bb0aa7..b7169b1 100644
--- a/configs/ethernut5_defconfig
+++ b/configs/ethernut5_defconfig
@@ -58,6 +58,7 @@
 CONFIG_ENV_IS_IN_SPI_FLASH=y
 CONFIG_ENV_SPI_MAX_HZ=15000000
 CONFIG_SYS_RELOC_GD_ENV_ADDR=y
+CONFIG_NET_RETRY_COUNT=20
 CONFIG_DM=y
 CONFIG_CLK=y
 CONFIG_CLK_AT91=y