adt_installer: allow specifying relative target sysroot paths

adt_installer: allow specifying relative target sysroot paths

If one specifies a relative target sysroot path, then he/she must always
be in the same directory in order to be able to compile.

With this patch, adt_installer will automatically convert user
supplied relative paths to absolute.

[YOCTO #4955]

(From OE-Core rev: 1abf13cb035fa9e02f0c6a1c6f3524b649d2a701)

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>