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/corvus_defconfig b/configs/corvus_defconfig
index 4688e0a..6ea76db 100644
--- a/configs/corvus_defconfig
+++ b/configs/corvus_defconfig
@@ -54,6 +54,7 @@
 CONFIG_ENV_IS_IN_NAND=y
 CONFIG_SYS_REDUNDAND_ENVIRONMENT=y
 CONFIG_SYS_RELOC_GD_ENV_ADDR=y
+CONFIG_NET_RETRY_COUNT=20
 CONFIG_CLK=y
 CONFIG_CLK_AT91=y
 CONFIG_DFU_NAND=y