base-files: enable get_mac_ascii() to handle redundant strings

base-files: enable get_mac_ascii() to handle redundant strings

The function get_mac_ascii() will fail when there are two or more
same MAC address variable names in the mtd partition. Only retain
the first variable to workaround this rare situation.

Fixes: https://github.com/openwrt/openwrt/issues/17236
Signed-off-by: Shiji Yang <yangshiji66@qq.com>
Link: https://github.com/openwrt/openwrt/pull/17262
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>
(cherry picked from commit 3410ad2f40c085e5db7a71f6ae4b80734826a1e3)