new package libusb1

This is a package for the libusb-1.0 shared library (MXE package name
"libusb1") used by various projects. It is not to be confused with the
older libusb-0.1 library (MXE package name "libusb") which is a
completely different library with different API. Both libs are in
wide-spread use and can be installed in parallel.
mxe-activate: get targets from Makefile
mxe-activate: clear env vars on make invocation
mxe-activate: unset env vars for interactive use
mxe-activate: get targets from Makefile
Merge branch 'stable'
doc update: re-align requirements install commands
Merge pull request #227 from uwehermann/libusb1

new package libusb1
add package pkgconf (optional native build)
doc update: add EDITOR to env whitelist for interactive sessions
doc update: add MXE to env whitelist for interactive sessions