mingetty: fix do_package warning

mingetty: fix do_package warning

Reproduce steps(Under the same project dir):
1. enable DISTRO_FEATURES usrmerge,  bitbake mingetty
2. disable DISTRO_FEATURES usrmerge, bitbake mingetty

Result in step 2:
WARNING: mingetty-1.08-r3 do_package: mingetty: NOT adding alternative provide /usr/sbin/getty: /usr/sbin/mingetty does not exist
WARNING: mingetty-1.08-r3 do_package: QA Issue: mingetty: Files/directories were installed but not shipped in any package:
  /sbin
  /usr/sbin

In step1, Line SBINDIR=/sbin is replaced to SBINDIR=/usr/sbin, in step2,
since do_fetch does not rerun, Makefile still has SBINDIR=/usr/sbin, so
sed not works as expected, SBINDIR still equal to /usr/sbin when disable
usrmerge. And cause above two warnings.

Signed-off-by: Changqing Li <changqing.li@windriver.com>
Signed-off-by: Steve Sakoman <steve@sakoman.com>