package: fix fallout from checking kconfig config files

Those checks should only be done when the package is actually enabled.

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

package/busybox/busybox.mk

@@ -225,7 +225,7 @@ endef
 
 # Checks to give errors that the user can understand
 # Must be before we call to kconfig-package
-ifeq ($(BR_BUILDING),y)
+ifeq ($(BR2_PACKAGE_BUSYBOX)$(BR_BUILDING),yy)
 ifeq ($(call qstrip,$(BR2_PACKAGE_BUSYBOX_CONFIG)),)
 $(error No BusyBox configuration file specified, check your BR2_PACKAGE_BUSYBOX_CONFIG setting)
 endif

package/uclibc/uclibc.mk

@@ -476,7 +476,7 @@ endef
 
 # Checks to give errors that the user can understand
 # Must be before we call to kconfig-package
-ifeq ($(BR_BUILDING),y)
+ifeq ($(BR2_PACKAGE_UCLIBC)$(BR_BUILDING),yy)
 ifeq ($(call qstrip,$(BR2_UCLIBC_CONFIG)),)
 $(error No uClibc configuration file specified, check your BR2_UCLIBC_CONFIG setting)
 endif