Handle absolute source file paths

We used to fail there because we unconditionally prefixed the relative directory where it was referenced.
Tested in various scenarios:
- obj=build
- obj=../obj
- obj=$PWD/../obj
- obj=/some/other/absolute/path
- obj=/./some/other/absolute/path

In-tree relative paths still work as before, the only change in behaviour is when a source file name (as specified in one of the *-y variables) starts with "/".

Signed-off-by: Patrick Georgi <patrick@georgi-clan.de>
Acked-by: Patrick Georgi <patrick@georgi-clan.de>
git-svn-id: svn://svn.coreboot.org/coreboot/trunk@6606 2b7e53f0-3cfb-0310-b3e9-8179ed1497e1