[FREELDR] Improvements for the RamDisk support.
- Implement support for the "RDIMAGELENGTH" and "RDIMAGEOFFSET" boot
options. Fixes CORE-15432.
- Separate the initialization of the global gInitRamDiskBase /
gInitRamDiskSize variables from the FreeLdr command-line, and the
actual initialization of the internal variables of the RamDisk.
The latter are initialized via calls to RamDiskInitialize().
- Implement 'SeekRelative' mode in RamDiskSeek().
- Make RamDiskLoadVirtualFile() internal function that gets called by
RamDiskInitialize(), and we use the latter in the NT loader instead.