ChaCha20 implementation for encryption

Add an implementation in 8051 assembler for the generation of a
ChaCha20 keystream and an encryption function using the keystream
in C.

A test function using the example from RFC7539 is provided in
chacha20_test.

Keys, nonce, and block counter and the pointers to the input plaintext,
its length and the output pointer are provided via a struct chacha20,
which also holds the working state of the ChaCha20 algorithm

Makefile

@@ -66,6 +66,7 @@ create_build_dir:
 	mkdir -p "$(BUILDDIR)/uip"
 	mkdir -p "$(BUILDDIR)/httpd"
 	mkdir -p "$(BUILDDIR)/machine"
+	mkdir -p "$(BUILDDIR)/crypto"
 
 # Every machine is a pair machine/<MACHINE>.c + machine/<MACHINE>.h. Selecting
 # them by name means no #if/#elif chain, so an unknown or empty $MACHINE is
@@ -94,7 +95,8 @@ SRCS = \
 	ntp.c \
 	ntp_cli.c \
 	syslog.c \
-	udp_apps.c
+	udp_apps.c \
+	crypto/chacha20.c
 
 # RTL837x
 SRCS += \
@@ -183,7 +185,7 @@ $(BUILDDIR)/%.rel: %.asm $(CCFLAGS_STAMP) | create_build_dir
 	${ASM} ${AFLAGS} -o $@ $<
 #	mv -f $(addprefix $(basename $^), .lst .rel .sym) .
 
-$(BUILDDIR)/rtlplayground.ihx: $(OBJS) $(BUILDDIR)/crtbank.rel $(BUILDDIR)/crc16.rel
+$(BUILDDIR)/rtlplayground.ihx: $(OBJS) $(BUILDDIR)/crtbank.rel $(BUILDDIR)/crc16.rel $(BUILDDIR)/crypto/chacha_8051.rel
 	$(CC) $(CC_FLAGS) --xram-size 49151 -Wl-bHOME=0x00000 -Wl-bBANK1=0x14000 -Wl-bBANK2=0x24000 -Wl-bBANK3=0x34000 -Wl-r -o $@ $^
 
 $(BUILDDIR)/rtlplayground.img: $(BUILDDIR)/rtlplayground.ihx