Fix bug due to interaction of partition table & string interning

A user reported that the previous commit (03fca656d70) — which added a partition table for USB drive booting — was causing SectorLISP to crash on the input ``` (CONS (QUOTE AAA) NIL) ``` This turned out to be because the SectorLISP code interns new tokens starting at an area where it sees two null bytes. The newly added "partition table" entry for a bootable partition contains multiple `00`'s in a row, & was followed by code. The code was clobbered. This commit moves the bootable partition table entry towards the end of the partition table, which is safe to clobber.