net: Kconfig: add CONFIG_WGET symbol

Let net/wget.c and net/lwip/wget.c depend on CONFIG_WGET, and
cmd/wget.c depend on CONFIG_CMD_WGET. This way, the wget code
can be used regardless of whether the wget command is available.

Signed-off-by: Adriano Cordova <adrianox@gmail.com>
Reviewed-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
diff --git a/cmd/Kconfig b/cmd/Kconfig
index 6368336..8f3ad94 100644
--- a/cmd/Kconfig
+++ b/cmd/Kconfig
@@ -2115,11 +2115,8 @@
 
 config CMD_WGET
 	bool "wget"
-	depends on CMD_NET
 	default y if SANDBOX
-	select PROT_TCP if NET
-	select PROT_TCP_LWIP if NET_LWIP
-	select PROT_DNS_LWIP if NET_LWIP
+	select WGET
 	help
 	  wget is a simple command to download kernel, or other files,
 	  from a http server over TCP.