polarssl: Fix build failures due to PKG_NAME != dir name

Packages that depend on PolarSSL fail to build because polarssl's InstallDev
section never actually gets executed because (prior to this patch) the package
name does not match the subdir the package is in (presumably due to upstream
name change).  As a workaround I have changed the package name back to
polarssl and used a new variable SRC_PKG_NAME for the purposes of downloading
the upstream tarball and creating PKG_BUILD_DIR.

Signed-off-by: Daniel Dickinson <openwrt@daniel.thecshore.com>

SVN-Revision: 46683