package_ipk: Use preferred form of --force-postinstall

opkg accepts both - and _ in its options but use the help text
preferred format. From https://github.com/yoctoproject/poky/pull/14
from Philipp-Alexander Blum <blum@gessler.de>.

(From OE-Core rev: 4bea64347407dfb1eb46dacffd05af3bd200b419)

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

meta/classes-global/package_ipk.bbclass

@@ -17,7 +17,7 @@ PKGWRITEDIRIPK = "${WORKDIR}/deploy-ipks"
 # Program to be used to build opkg packages
 OPKGBUILDCMD ??= 'opkg-build -Z zstd -a "${ZSTD_DEFAULTS}"'
 
-OPKG_ARGS += "--force_postinstall --prefer-arch-to-version"
+OPKG_ARGS += "--force-postinstall --prefer-arch-to-version"
 OPKG_ARGS += "${@['', '--no-install-recommends'][d.getVar("NO_RECOMMENDATIONS") == "1"]}"
 OPKG_ARGS += "${@['', '--add-exclude ' + ' --add-exclude '.join((d.getVar('PACKAGE_EXCLUDE') or "").split())][(d.getVar("PACKAGE_EXCLUDE") or "").strip() != ""]}"