SDK: allow toolchain installation from another directory

SDK: allow toolchain installation from another directory

This patch will allow one to run the installer from another directory
than the one where it's actually located.

Suppose the installer is in /home/user/test/my/sdk and the current
directory is in a different place. With this patch, one can run the
installer like this:

$ sh ~/test/my/sdk/poky-eglibc-x86_64-arm-toolchain-gmae-1.2+snapshot-20120920.sh

[YOCTO #3135]

(From OE-Core rev: 3c7aac33cb63dc63b989db4e9d7389a7f4d3c18d)

Signed-off-by: Laurentiu Palcu <laurentiu.palcu@intel.com>
Signed-off-by: Saul Wold <sgw@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>