apply-patches.sh: make scan_patchdir work recursively

scan_patchdir is called recursively. For this to work properly, the
variable path which is set to $1 at the very beginning must be local not
global.

A test case is to set BR2_GLOBAL_PATCH_DIR to 'mypatches' and having the
following tree in the buildroot root:

$ find mypatches/
mypatches/
mypatches/busybox
mypatches/busybox/subdir.patch
mypatches/busybox/subdir.patch/busybox-0001-abc.patch
mypatches/busybox/busybox-0002-def.patch
mypatches/busybox/asubdir.patch
mypatches/busybox/asubdir.patch/busybox-0003-xyz.patch

When running 'make busybox-dirclean busybox-patch' originally, you'd get:

    Applying busybox-0003-xyz.patch using patch:

    Applying busybox-0002-def.patch using patch:
    Error: missing patch file
    mypatches/busybox/asubdir.patch/busybox-0002-def.patch

While with this fix:

    Applying busybox-0003-xyz.patch using patch:

    Applying busybox-0002-def.patch using patch:

    Applying busybox-0001-abc.patch using patch:

This fixes bug #6434 (https://bugs.busybox.net/show_bug.cgi?id=6434

)

Signed-off-by: Daniel Mentz <daniel@exxm.de>
[Thomas: update commit message with test case]
Signed-off-by: Thomas De Schampheleire <thomas.de.schampheleire@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>