net: split net into net{,-common,-legacy,-lwip}

Make net.h a wrapper which includes net-common.h and either
net-legacy.h or net-lwip.h based on NET_LWIP. The function
copy_filename() can be useful when NET_LWIP is enabled, therefore
move it out of net/net.c which is built only when networking choice
is NET and create a new file net/net-common.c.

Signed-off-by: Jerome Forissier <jerome.forissier@linaro.org>
Acked-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>
diff --git a/net/Makefile b/net/Makefile
index 33c82b5..34fe501 100644
--- a/net/Makefile
+++ b/net/Makefile
@@ -42,3 +42,5 @@
 CFLAGS_eth_common.o += -Wno-format-extra-args
 
 endif
+
+obj-y += net-common.o
diff --git a/net/net-common.c b/net/net-common.c
new file mode 100644
index 0000000..a7f767d
--- /dev/null
+++ b/net/net-common.c
@@ -0,0 +1,13 @@
+// SPDX-License-Identifier: GPL-2.0
+
+void copy_filename(char *dst, const char *src, int size)
+{
+	if (src && *src && (*src == '"')) {
+		++src;
+		--size;
+	}
+
+	while ((--size > 0) && src && *src && (*src != '"'))
+		*dst++ = *src++;
+	*dst = '\0';
+}
diff --git a/net/net.c b/net/net.c
index 64bcf69..f47e9fb 100644
--- a/net/net.c
+++ b/net/net.c
@@ -1693,18 +1693,6 @@
 	ip->udp_xsum = 0;
 }
 
-void copy_filename(char *dst, const char *src, int size)
-{
-	if (src && *src && (*src == '"')) {
-		++src;
-		--size;
-	}
-
-	while ((--size > 0) && src && *src && (*src != '"'))
-		*dst++ = *src++;
-	*dst = '\0';
-}
-
 int is_serverip_in_cmd(void)
 {
 	return !!strchr(net_boot_file_name, ':');