kernel-install: rename $BOOT_DIR[_ABS] to $ENTRY_DIR[_ABS]

"BOOT" is misleading, because it sounds like this refers to /boot or $BOOT,
when in fact it refers to some subdirectory. Those variable names are purely
interal, so we can change them. $BOOT_DIR_ABS was used in NEWS, but it should
not be (because it is an internal detail), so the old NEWS entry is reworded to
use "entry directory".