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.

(From OE-Core rev: 12539d529c6af3d4a56ff4f1e1420e7e4d169804)

Signed-off-by: Changqing Li <changqing.li@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

meta/recipes-extended/mingetty/mingetty_1.08.bb

@@ -15,7 +15,7 @@ EXTRA_OEMAKE = "CC='${CC}' \
                 CFLAGS='${CFLAGS} -D_GNU_SOURCE'"
 
 do_install(){
-    sed -i -e "s;SBINDIR=/sbin;SBINDIR=$base_sbindir;"  ${S}/Makefile
+    sed -i -e "/^SBINDIR=/c SBINDIR=$base_sbindir"  ${S}/Makefile
     install -d ${D}${mandir}/man8 ${D}/${base_sbindir}
     oe_runmake install DESTDIR=${D}
 }