busybox: Add -Wno-error=format-nonliteral to compiler flags

busybox implements some of its own string handling that triggers
warnings with -Wformat-nonliteral.
This has been verified to be harmless in 1.31.1.

Removing -Werror=format-nonliteral here allows us to use
-Werror=format-nonliteral globally in OPTIMIZE_FOR=security mode
while keeping busybox building.

Signed-off-by: Bernhard Rosenkränzer <bernhard.rosenkraenzer.ext@huawei.com>