devtools: fix build test config inheritance from env

devtools: fix build test config inheritance from env

The variables DPDK_MESON_OPTIONS, PATH, PKG_CONFIG_PATH,
CPPFLAGS, CFLAGS and LDFLAGS can be customized in the config file
loaded by devtools/load-devel-config at each build.
The configuration can be adjusted per target thanks to the value set
in the DPDK_TARGET variable.

PKG_CONFIG_PATH is specific to each target, so it must be empty
before configuring each build from the file according to DPDK_TARGET.
Inheriting a default PKG_CONFIG_PATH for all targets does not make sense
and is prone to confusion.

DPDK_MESON_OPTIONS might take a global initial value from environment
to customize a build test from the shell. Example:
	DPDK_MESON_OPTIONS="b_lto=true"
Some target-specific options can be added in the configuration file:
	DPDK_MESON_OPTIONS="$DPDK_MESON_OPTIONS kernel_dir=$MYKERNEL"

Fixes: 272236741258 ("devtools: load target-specific compilation environment")
Cc: stable@dpdk.org
Signed-off-by: Thomas Monjalon <thomas@monjalon.net>
Acked-by: Bruce Richardson <bruce.richardson@intel.com>
Acked-by: Ferruh Yigit <ferruh.yigit@intel.com>
Tested-by: Jerin Jacob <jerinj@marvell.com>
Acked-by: David Marchand <david.marchand@redhat.com>