Fix loading of boot configuration

[phil-opp]

Fixes various issues in the boot config file loading introduced in #326:

• Test that boot config is actually loaded (through a sentinel value)
  • before, we only verified that no panic occurs, but we didn't check the values
  • the new sentinel value is passed through the boot info as a hidden field
• Use correct file name when loading boot config file
• Include boot config when using TFTP booting
  • looks like we forgot to update this part
• Fix: Load kernel first, then use same boot mode for config file
  • before, we always tried to load the config file from disk
• Fix: Consider config file when initializing frame allocator
  • We still considered the ramdisk file as the last used frame when initializing the frame allocator, so we overwrote the config file.