allow to change java memory limits

useful for people with not so much RAM, as well as
for people with too much RAM

example usage on linux:
    export CUSTOM_JAVA_MEMORY_LIMIT=2048M

Change-Id: Ic84ea6010cc093a8f141f484beaeae525b227482
allow to modify java target source

example usage on linux:
    export CUSTOM_JAVA_TARGET_SOURCE=1.7

Change-Id: Idf607e6494f61bbe406354c89ca304d8bac1d446
Increase the maximum javac heap size

Change-Id: Id3bbc03bdb9155289a19ac4ba3099b49cf4c59d6