build: enable a developer mode setting

build: enable a developer mode setting

To allow support for additional build checks and tests only really
relevant for developers, we add support for a developer mode option to
DPDK. The default, "auto", value for this enables developer mode if a
".git" folder is found at the root of the source tree - as was the case
with the previous "make" build system. There is also support for
explicitly enabling or disabling this option using "meson configure" if
so desired.

Signed-off-by: Bruce Richardson <bruce.richardson@intel.com>