asterisk-chan-dongle: bump + cleanup

asterisk-chan-dongle: bump + cleanup

Bump dongle plugin to latest git revision. PKG_SOURCE_SUBDIR and
PKG_BUILD_DIR are removed because defaults are fine. PKG_VERSION is
replaced by PKG_SOURCE_DATE, which is in line with OpenWrt
recommendation.

Soft deps are removed as per request in issue #536.

iconv handling is updated. Upstream now lets us specify the include
directory, so that makes the patch simpler. The iconv lib detection is
also updated so that it never picks the libc iconv. This way we get the
same iconv handling independent of the libc in use. Now it'll be always
iconv-stub or iconv-full.

libsqlite3 dep is made explicit. There was always a dep on it, but it
was implicit via asterisk. When the dep is made explicit, the dongle
package picks up on the sqlite3 ABI_VERSION, which is nice.

Signed-off-by: Sebastian Kemper <sebastian_ml@gmx.net>