curl: skip SSL certificate verification

Skip SSL certificate verification. We validate the downloaded content
with m5sum which is sufficient for time being.
strip_tree.pl: skip .ko files

Don't try to strip .ko files.
strip_tree.pl: add missing OBJCOPY check

Add missing OBJCOPY check.
setup: fix image names with appended DTB

Board name should be present also in images with appended DTB.
setup: pkg_native_pre: add missing PKG_BUILD_NAME variable

PKG_BUILD_NAME should be propagated to the native build script.