treewide: use name in define and eval lines

For consistency, use full name instead of $(PKG_NAME) in define and eval
lines for all packages.

I've seen reviews that asked to do this before, and I am asking the same
during reviews now. To avoid this in the future, fix this treewide so
when people use existing packages as example, we will not have to
request this change anymore.

This makes all packages consistent with both LEDE and OpenWrt base
repositories.

Signed-off-by: Stijn Tintel <stijn@linux-ipv6.be>

mail/ssmtp/Makefile

@@ -21,7 +21,7 @@ include $(INCLUDE_DIR)/package.mk
 
 TARGET_CFLAGS += $(TARGET_CPPFLAGS)
 
-define Package/$(PKG_NAME)
+define Package/ssmtp
 	SECTION:=mail
 	CATEGORY:=Mail
 	DEPENDS:=+libopenssl
@@ -29,14 +29,14 @@ define Package/$(PKG_NAME)
 	URL:=http://packages.debian.org/ssmtp
 endef
 
-define Package/$(PKG_NAME)/description
+define Package/ssmtp/description
 A secure, effective and simple way of getting mail off a system to your mail hub.
 Mail is simply forwarded to the configured mailhost, no daemons running in the background.
 Extremely easy configuration.
 
 endef
 
-define Package/$(PKG_NAME)/conffiles
+define Package/ssmtp/conffiles
 /etc/ssmtp/ssmtp.conf
 /etc/ssmtp/revaliases
 endef
@@ -47,7 +47,7 @@ CONFIGURE_VARS += \
 CONFIGURE_ARGS += \
 	--enable-ssl
 
-define Package/$(PKG_NAME)/install
+define Package/ssmtp/install
 	$(INSTALL_DIR) $(1)/etc/ssmtp
 	$(INSTALL_CONF) $(PKG_BUILD_DIR)/ssmtp.conf $(1)/etc/ssmtp/
 	$(INSTALL_DATA) $(PKG_BUILD_DIR)/revaliases $(1)/etc/ssmtp/
@@ -56,4 +56,4 @@ define Package/$(PKG_NAME)/install
 	ln -s /usr/sbin/ssmtp $(1)/usr/sbin/sendmail
 endef
 
-$(eval $(call BuildPackage,$(PKG_NAME)))
+$(eval $(call BuildPackage,ssmtp))