package/pkg-generic.mk, support/scripts/fix-rpath: fix per-package regexp

package/pkg-generic.mk, support/scripts/fix-rpath: fix per-package regexp

Commit c4e6d5c8be6ada8e7c60950e3b499c55d48761cb ("core: implement
per-package SDK and target") had a mistake on the regexp that is used
to match $(PER_PACKAGE_DIR)/<something>/, and due to this, the regexp
was never matched.

The + sign in [^/]+ which was suggested by Yann E. Morin during the
review of the per-package patch series (instead of [^/]*) needs to be
escaped to be taken into account correctly. Without this, the regexp
doesn't match, and the replacement is not done, causing:

 (1) For the libtool fixup in pkg-generic.mk, the lack of replacement
     causes libtool .la files to not be tweaked as expected, which it
     turn causes build failures reported by the autobuilder.

 (2) For the fix-rpath, the RPATH of host binaries in the SDK were not
     correct.

Interestingly, we have the same regexp in
support/scripts/check-host-rpath, but here the + sign does not need to
be escaped.

Fixes:

  http://autobuild.buildroot.net/results/d4d996f3923699e266afd40cc7180de0f7257d99/ (libsvg-cairo)
  http://autobuild.buildroot.net/results/56330f86872f67a2ce328e09b4c7b12aa835a432/ (bind)
  http://autobuild.buildroot.net/results/9e0fc42d2c9f856b92954b08019b83ce668ef289/ (ibrcommon)
  and probably a number of other similar issues

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>