docker-engine: needs pkg-config

docker-engine: needs pkg-config

Fixes:
http://autobuild.buildroot.org/results/fcc/fcc32f6cdc46f354a016a9b137d4f51d4054cdb6/
http://autobuild.buildroot.org/results/083/0832b7be86c525522e2caed66246d5aeb365afb2/
http://autobuild.buildroot.org/results/ede/ede0ddf0c5685ece8443a9b1bbd5c63b4d98af33/

pkg-config is used for some of the docker-engine dependencies (at least
libseccomp and libsystemd), so make sure it is available.

The make.sh script used to configure docker-engine uses the PKG_CONFIG
variable, but the libseccomp-golang go module simply uses whatever
pkg-config binary it finds in the path at build time, so also pass
TARGET_MAKE_ENV to ensure our cross-pkg-config is used.

For consistency and robustness against future changes, use both approaches
for the configure and build steps.

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>