initrd: Tell make to use makefiles from the source tree

initrd: Tell make to use makefiles from the source tree

Otherwise, make uses makefiles from the system installed ones at
/usr/share/mk, therefore, the updated bsd.prog.mk has no effect until
being installed to the system.

Note that the variables assigned by make (i.e., make ... var=value)
will override the shell enviroment variables (e.g., var=value shell
command).  So make sure the make variable assignments (e.g., DESTDIR)
are put after the make.

Also fix mkinitrd.sh to remove the used temporary initrd image file.