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".

(cherry picked from commit d271c5d3)