uboot-tools: fix static linking with OpenSSL

Add a patch to U-Boot to use pkg-config to get the OpenSSL link flags
when pkg-config is available. This allows to make sure that static
linking works properly. The patch requires adding $(TARGET_MAKE_ENV)
so that the Buildroot provided pkg-config is used instead of the host
one.

The U-Boot patch has been submitted upstream.

Fixes:

  http://autobuild.buildroot.org/results/a9f/a9f316cd076a74b8730ce4cdcdb8176da4ed9eb3/



Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Tested-by: Gergely Imreh <imrehg@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>