tools/edbg: escape values set by 'env'

[leandrolanzieri]

Contribution description

Right now, for instance, if a path with spaces exists in the PATH environmental variable, the build process of edbg fails. This PR escapes the values passed to env in the Makefile of edbg (I'm not quite sure if this is also needed for PKG_BULDDIR).

Testing procedure

Set either PATH or TERM variables to values that contain spaces and try to build edbg (e.g. by ensuring it is not in dist/tools/edbg/edbg and trying to flash some board that uses it, like samr21-xpro).

Issues/PRs references

None

[jcarrano]

Steps to test:

In the RIOT root, run:

$ mkdir -p ~/"a b"
$ rm -f dist/tools/edbg/edbg
$ PATH=$PATH:~/"a b" BOARD=samr21-xpro make -C examples/hello-world flash-only

Fails in master, works with this patch.

[leandrolanzieri]

@jcarrano Thanks for reviewing