package/jpeg: convert to the virtual-package infrastructure

Since this package is implemented via a choice rather than the usual
separate-package providers, we only need BR2_PACKAGE_HAS_JPEG to be
always defined when the 'jpeg' package is selected.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Eric Le Bihan <eric.le.bihan.dev@free.fr>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Cc: Thomas De Schampheleire <patrickdepinguin@gmail.com>
Cc: Mike Zick <minimod@morethan.org>
Reviewed-by: Samuel Martin <s.martin49@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>

package/jpeg/Config.in

@@ -30,6 +30,9 @@ config BR2_PACKAGE_JPEG_TURBO
 
 endchoice
 
+config BR2_PACKAGE_HAS_JPEG
+	def_bool y
+
 config BR2_PACKAGE_PROVIDES_JPEG
 	string
 	default "libjpeg"    if BR2_PACKAGE_LIBJPEG

package/jpeg/jpeg.mk

@@ -4,14 +4,4 @@
 #
 ################################################################################
 
-JPEG_SOURCE =
-JPEG_DEPENDENCIES = $(call qstrip,$(BR2_PACKAGE_PROVIDES_JPEG))
-
-ifeq ($(JPEG_DEPENDENCIES),)
-define JPEG_CONFIGURE_CMDS
-	echo "No JPEG implementation defined. Configuration error"
-	exit 1
-endef
-endif
-
-$(eval $(generic-package))
+$(eval $(virtual-package))