ldlinux: Loading a config file should cause re-initialisation

ldlinux: Loading a config file should cause re-initialisation

There are a number of initialisation steps that need to be performed
*every* time a config file is loaded. Reload ldlinux.c32 so that we
can re-initialise the environment whenever a new config file is
loaded. This involves unloading all the modules that have been loaded
since ldlinux.c32. Luckily the list of loaded modules is sorted by
load order, which means it's trivial to "pop" them from the front of
the list.

Signed-off-by: Matt Fleming <matt.fleming@intel.com>