solenv: generate the ifdefs in native-code.py in a more elegant fashion

Commit 369b84e847802d77e7469f4ed71400c20e23039f (We don't build
libxsec_xmlsec for Android currently, 2014-08-13) added
platform-dependent guards to have components only on Android or only not
on Android. Generalize that a bit more, so adding new Android-only
components is a single line of code change.

Change-Id: I54711ffcdf64c4ddd986e2f59129797462dcfe58