[Buildroot] allow build infrastructure to pick up installed meson tool

[Norbert Lange]

This is v3 of the patchset, only patch 1 got modifications.

Patch 1 will install a cross-compilation.conf as part of the toolchain
target, removing this logic from the meson package.

Patch 2 + Patch 3 allow picking up meson + ninja from the system,
this functionality is currently disabled.

Patch 4 moves some common functionality of the support tool
handling into a helper script.

[v2->v3]
-   rebase
-   support overriding [C,CXX,LD]FLAGS in package
-   place most logic into a HOST_MESON_CROSSCONFIG_SED variable
-   further refactoring and simplification