package/btrfs-progs: install to staging directory

package/btrfs-progs: install to staging directory

Other packages when building against btrfs-progs require headers in the
staging directory under /usr/include/btrfs. This patch enables
installing btrfs-progs to the staging directory to enable other packages
to build against the btrfs-progs headers.

Signed-off-by: Christian Stewart <christian@paral.in>
[Thomas: take into account the BR2_STATIC_LIBS case, by adding the
relevant BTRFS_PROGS_INSTALL_STAGING_OPTS variable, as well as a patch
to make sure "install-static" installs the static library and the
headers.]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>