package/meson: install cross-compilation.conf during toolchain install

package/meson: install cross-compilation.conf during toolchain install

package/meson installs a cross-compilation.conf file in
$(HOST_DIR)/etc/meson, via TARGET_FINALIZE_HOOKS.

package/pkg-cmake.mk installs a toolchainfile.cmake in
$(HOST_DIR)/share/buildroot, via TOOLCHAIN_POST_INSTALL_STAGING_HOOKS.

Both files have a similar concept, they describe some flags/paths needed for
compilation using respective build systems. One difference is that the meson
file is added for external compilation, from the SDK, while the cmake file
is used internally in Buildroot.

The 'problem' of using TARGET_FINALIZE_HOOKS for the meson file, is that it
installs a 'host' file from target-finalize, which is conceptually incorrect
since not just TARGET_DIR but also HOST_DIR is "regenerated" on a subsequent
'make' when everything was already built (i.e. only target-finalize is run).

This can easily be fixed, by using the same hook as cmake uses, i.e.
TOOLCHAIN_POST_INSTALL_STAGING_HOOKS.

Note that actually even for cmake, TOOLCHAIN_POST_INSTALL_STAGING_HOOKS is
not the best hook to install a host file. A better hook would have been
TOOLCHAIN_POST_INSTALL_HOOKS, but this triggers only for 'host' packages,
and 'toolchain' is treated as a 'target' package.

Also, the hook (and therefore also the definition of
PKG_MESON_INSTALL_CROSS_CONF) is moved to pkg-meson.mk, again to make it
more similar to how it's done for cmake. Otherwise check-package
complains that the meson package is setting variables that don't start
with MESON_.

Signed-off-by: Thomas De Schampheleire <thomas.de_schampheleire@nokia.com>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>