Fix Java detection on some Linux distributions

On some Linux distributions (spotted here on OpenMandriva Lx, but I'm
pretty sure some others do the same thing), "which javac" returns
/usr/bin/javac, which is a symlink to "../../etc/alternatives/javac",
which in turn points at whatever the JDK the user picked as his default.

Given "../../etc/alternatives/javac" is a relative, symlink, the next
iteration of LSLINE=$(ls -l "$JAVAC") fails (no
../../etc/alternatives/java relative to the build directory), causing
tools.jar not to be found.

Using readlink -f should work in all cases (while also simplifying the
script a bit).

Change-Id: I96afcf8e6460861bf8bb849dae05c27c42b91e84
Signed-off-by: Bernhard Rosenkränzer <Bernhard.Rosenkranzer@linaro.org>