busybox: enable CONFIG_FEATURE_AWK_LIBM by default

busybox: enable CONFIG_FEATURE_AWK_LIBM by default

It was required by tzselect which is provided by eglibc utils.
The eglibc is a fundamental package, so enable math functions
of awk is reasonable.

Here is the error if we not enable that:
...
root@qemuarm:~# tzselect -c +4852+00220
awk: cmd. line:39: Math support is not compiled in
...

[YOCTO #6544]

Signed-off-by: Hongxu Jia <hongxu.jia@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>