package/pkg-generic.mk: really make DL_SUBDIR inheritance work

While bumping kodi, we figured out that the kodi-texturepacker and
kodi-jsonschemabuilder were both re-downloading the main Kodi tarball,
even though they contain:

KODI_JSONSCHEMABUILDER_DL_SUBDIR = kodi
KODI_TEXTUREPACKER_DL_SUBDIR = kodi

Both are host packages, and turns out that changing those variables to
HOST_ ones made the download sharing work.

Commit efa7712b092950c92f994e2ee30c120a64e5451b ("package/pkg-generic:
host variant inherits target download settings") introduced
inheritance of host variables from target variables from a number of
variables, including DL_SUBDIR. But it missed the fact that earlier in
pkg-generic.mk, the following line was defined:

  $(2)_DL_SUBDIR ?= $$($(2)_RAWNAME)

So, when this later code kicked in:

 ifndef $(2)_DL_SUBDIR
  ifdef $(3)_DL_SUBDIR
   $(2)_DL_SUBDIR = $$($(3)_DL_SUBDIR)
  endif
 endif

In fact it never did anything because $(2)_DL_SUBDIR would never be
undefined. This commit fixes this issue by properly adjusting the
logic to inherit the value of the target variable when it exists, or
defaulting to $$($(2)_RAWNAME) otherwise.

Cc: Yann E. MORIN <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>

package/pkg-generic.mk

@@ -525,7 +525,6 @@ endif
 
 $(2)_BASENAME	= $$(if $$($(2)_VERSION),$(1)-$$($(2)_VERSION),$(1))
 $(2)_BASENAME_RAW = $$(if $$($(2)_VERSION),$$($(2)_RAWNAME)-$$($(2)_VERSION),$$($(2)_RAWNAME))
-$(2)_DL_SUBDIR ?= $$($(2)_RAWNAME)
 $(2)_DL_DIR = $$(DL_DIR)/$$($(2)_DL_SUBDIR)
 $(2)_DIR	=  $$(BUILD_DIR)/$$($(2)_BASENAME)
 
@@ -538,6 +537,8 @@ endif
 ifndef $(2)_DL_SUBDIR
  ifdef $(3)_DL_SUBDIR
   $(2)_DL_SUBDIR = $$($(3)_DL_SUBDIR)
+ else
+  $(2)_DL_SUBDIR = $$($(2)_RAWNAME)
  endif
 endif