et4x00.conf: use mips32el-nf tune features

This takes care of omitting the fpu-hard tune feature,
and puts soft float packages in their own (mips32el-nf) feed.
Which is necessary, because hard/soft float feeds cannot be shared.

Based on: http://sourceforge.net/p/openpli/openpli-oe-core/ci/fe4f3996b6c95c30f00fb7097d1b996e14c73187/
remove preferred linux-libc-headers versions

just use the latest version, we cannot allow
different versions to be used, because
all machines share the same mips32el feed.

Based on: http://sourceforge.net/p/openpli/openpli-oe-core/ci/bdc98231b0d06f06d4c4fb1dde4d7d54c5988713/
Merge pull request #2 from athoik/mipsel-nf-tune

et4x00.conf: use mips32el-nf tune features