L4Linux: fix usage of l4_touch_xx (fix #682)

Use size in bytes, when touching memory via l4_touch_ro or l4_touch_rw, instead
of log2 size.
base-foc: remove links to obsolete initrd archives

Fix #686
Remove 'epit' from generic i.MX platform spec

Remove the 'epit' variable from the generic imx31 and imx53 specification,
and only add it to base-hw specific i.MX specs. Thereby the EPIT timer
library gets build for base-hw only.
Moreover, fix some const-ness issues in the platform_timer implementation
for the EPIT timer.
Fixes #688.