Fix platform_paths.prf paths to be easily usable also outside MMP_RULES/BLD_INF_RULES

Currently paths provided by platform_paths.prf are in a format suitable for directly adding to bld.inf or mmp files. There are however other use cases, such as exists checks for some headers which are currently hardcoded (see symbian.conf for example). Hardcoding platform dependent paths is bad, so the paths in platform_paths.prf should be usable for these checks. There are at least two problems with them currently that make it inconvenient:

1) They require prepending epocroot, but start with slash, so such prepend will create double slash, which creates a valid network path in windows if epocroot is just single slash.
2) Linux makefile based builds that use epocroot will prepend full epocroot already in platform_paths.prf.

Path format cannot be changed without seriously breaking lots of existing usage, so maybe a helper function to fix the paths for other use cases would be the answer.

Also fix any hardcoded platform dependent path usage to use platform_paths.prf.