contrib: add nettle rules

Dependency required by gnutls.

Based on VideoLan code by Rémi Denis-Courmont et al.
Refs: #46617

Change-Id: I12c458c389b52d3f10dbf30e9803e45074d53f83

contrib/src/nettle/SHA512SUMS

+edee4b7ecd34d7db8636650e59916c358a9be323128b4b7ad6eb4088a0a09a6744d0ebf382b23143fbcee3d40b148f43a54f25759141b16ba0befc3c46740b80  nettle-2.6.tar.gz

contrib/src/nettle/rules.mak

+# Nettle
+
+NETTLE_VERSION := 2.6
+NETTLE_URL := ftp://ftp.gnu.org/gnu/nettle/nettle-$(NETTLE_VERSION).tar.gz
+
+ifeq ($(call need_pkg,"nettle >= 2.6"),)
+PKGS_FOUND += nettle
+endif
+
+$(TARBALLS)/nettle-$(NETTLE_VERSION).tar.gz:
+	$(call download,$(NETTLE_URL))
+
+.sum-nettle: nettle-$(NETTLE_VERSION).tar.gz
+
+nettle: nettle-$(NETTLE_VERSION).tar.gz .sum-nettle
+	$(UNPACK)
+	$(UPDATE_AUTOCONFIG)
+	$(MOVE)
+
+DEPS_nettle = gmp $(DEPS_gmp)
+
+.nettle: nettle
+	cd $< && $(HOSTVARS) ./configure $(HOSTCONF)
+	cd $< && $(MAKE) install
+	touch $@