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 realpath and readlink where possible should work in all cases.

Change-Id: Ic60ac84a5b263dc1c1f2960092a7549d1024ed2e
Signed-off-by: Bernhard Rosenkraenzer <Bernhard.Rosenkranzer@linaro.org>