build: remove deprecated Meson functions

build: remove deprecated Meson functions

Starting in meson 0.56, the functions meson.source_root() and
meson.build_root() are deprecated and to be replaced by the [more
descriptive] functions: project_source_root()/global_source_root() and
project_build_root()/global_build_root(). Unfortunately, these new
replacement functions were only added in 0.56 release too, so to use
them we would need version checks for old/new functions to remove the
deprecation warnings.

However, the functions "current_build_dir()" and "current_source_dir()"
remain unaffected by all this, so we can bypass the versioning problem,
by saving off these values to "dpdk_source_root" and "dpdk_build_root"
in the top-level meson.build file

Bugzilla ID: 926
Cc: stable@dpdk.org
Signed-off-by: Bruce Richardson <bruce.richardson@intel.com>
Tested-by: Jerin Jacob <jerinj@marvell.com>