Add some TEMPORARY ext2 stuff, copied from FAT so far...
More ext2 work; file reading WORKS properly, including doubly indirect
Fix ATA buffer overflow bug!

When requesting 3 sectors, ata_read would miscalculate while trying
to optimize into 1 single read, and instead read 4 sectors, causing
a buffer overflow! It will now read first 2, then 1, as was
intended.
Fix ext2 file reading! Triply indirect blocks work.

Finally!
I have only tested this properly with 1 single large file so far.
(I've tested several smaller ones, but they don't need the triply
indirect pointer to be used.)
Print backtrace on assertion failures; also, allow scrollback after panic.
Add cksum command to kshell, for ext2 debugging
Very minor: print "Parsing MBRs" during boot.
Minor ext2 debug code removal (95% is still there, commented).