ext4fs: Fix pointer collision

ext4fs was using the device-specific pointer to keep its filesystem-
specific data.  This is a no-no, as the SATA device was using that pointer.

Add a new "fspriv" pointer for filesystem private data.
novena: ddr: Fix compile without DEBUG defined

Several bugs were found that arise from DEBUG not being defined.  We can
now calibrate DDR even when there is no screen to print to.
novena: keyboard: Correct help text environment var name

Correct the output of "help readkeys" so that it reflects the actual name
of the variable set.
ext2: Add ext2uuid command

The ext2uuid will return the UUID of a given ext2/3/4 partition.