ext-toolchain-wrapper: fix paths if executable was resolved by PATH

If ext-toolchain-wrapper or any symbolic link to it was resolved by PATH,
the wrapper takes the working directory to calculate the relative paths.

Now '/proc/self/exe' is used to resolve the absolute path to the toolchain
directory if the wrapper was called neither with a relative nor an absolute
path.

[Peter: fix off-by-one, swap value == var checks around]
Signed-off-by: Patrick Ziegler <patrick.ziegler@fh-kl.de>
Tested-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>