devtools: load target-specific compilation environment

devtools: load target-specific compilation environment

In order to re-use the same test environment as with
test-build.sh, the configuration file is loaded at each build,
after adjusting the variable DPDK_TARGET.

This is especially useful to set the variable PKG_CONFIG_PATH,
or define some meson options (without -D) in DPDK_MESON_OPTIONS.

The DPDK_TARGET values can be
	aarch64-*, powerpc64-*, x86_64-*.
The matching DPDK_TARGET values for test-build.sh are
	arm64-*, ppc_64-*, x86_64-*.
The advised expressions to use in the common configuration file are:
	if echo $DPDK_TARGET | grep -q '^a.*64-' ; then
	elif echo $DPDK_TARGET | grep -q '^p.*pc.*64' ; then
	elif echo $DPDK_TARGET | grep -q '^x86_64' ; then
	fi

Signed-off-by: Thomas Monjalon <thomas@monjalon.net>
Acked-by: Luca Boccassi <bluca@debian.org>