devtools: fix ABI compatibility check

devtools: fix ABI compatibility check

The new default-taget "linux" is introduced in v19.05-rc1
but not exist in before release such as v19.02 which have
default-target "linuxapp", there is no compatibility report
when run validate-abi.sh to check ABI compatibility between
v19.05-rc1 and v19.02, changed default-target from "linux"
to "linuxapp" in validate-abi.sh

Fixes: 218c4e68c1d9 ("mk: use linux and freebsd in config names")
Cc: stable@dpdk.org
Signed-off-by: Peng Huang <peng.huang@intel.com>
Acked-by: Neil Horman <nhorman@tuxdriver.com>
Acked-by: Bruce Richardson <bruce.richardson@intel.com>