linux-libc-headers: Don't do useless configuration
There's no point in configuring Linux kernel before
headers installation.
Moreover in come cases it might lead to problems on kernels
starting from 4.17 with architecture or platfrom sets
CROSS_COMPILE variable automatically, see [1].
Also note Buildroot doesn't do that kind of configuration as
well, see [2].
And while at it convert empty do_compile to noexec as well.
