net: Disable the format-extra-args warning

We will see warnings such as:
net/eth_common.c:57:61: warning: data argument not used by format string [-Wformat-extra-args]
        sprintf(enetvar, index ? "%s%daddr" : "%saddr", base_name, index);
                                              ~~~~~~~~             ^
With clang.  In this case we do not want to re-write our code to be less
compact as the above is intentional and readable.  Add a comment above
the disabling so that it's clear why we want that warning off.

Signed-off-by: Tom Rini <trini@konsulko.com>

net/Makefile

@@ -25,3 +25,8 @@ obj-$(CONFIG_CMD_PING) += ping.o
 obj-$(CONFIG_CMD_RARP) += rarp.o
 obj-$(CONFIG_CMD_SNTP) += sntp.o
 obj-$(CONFIG_CMD_NET)  += tftp.o
+
+# Disable this warning as it is triggered by:
+# sprintf(buf, index ? "foo%d" : "foo", index)
+# and this is intentional usage.
+CFLAGS_eth_common.o += -Wno-format-extra-args