zasm: allow duplicate const definition

This will allow interesting override scenarios, adding flexibility.

apps/zasm/README.md

@@ -115,6 +115,10 @@ allowed. An included file cannot have an `#inc` directive.
 **.equ**: Binds a symbol named after the first parameter to the value of the
           expression written as the second parameter. Example:
           `.equ foo 0x42+'A'`
+          
+          If the symbol specified has already been defined, no error occur and
+          the first value defined stays intact. This allows for "user override"
+          of programs.
 
 **.fill**: Outputs the number of null bytes specified by its argument, an
            expression. Often used with `$` to fill our binary up to a certain

apps/zasm/directive.asm

@@ -149,6 +149,13 @@ handleEQU:
 	ld	hl, DIREC_SCRATCHPAD
 	push	ix \ pop de
 	call	symRegisterConst	; A and Z set
+	jr	z, .end			; success
+	; register ended up in error. We need to figure which error. If it's
+	; a duplicate error, we ignore it and return success because, as per
+	; ".equ" policy, it's fine to define the same const twice. The first
+	; value has precedence.
+	cp	ERR_DUPSYM
+	; whatever the value of Z, it's the good one, return
 	jr	.end
 .badfmt:
 	ld	a, ERR_BAD_FMT

tools/tests/zasm/errtests.sh

@@ -55,5 +55,5 @@ chkerr "#inc foo" 19
 chkerr "ld a, 0x100" 20
 chkerr ".db 0x100" 20
 chkerr "#inc \"doesnotexist\"" 21
-chkerr ".equ foo 42 \\ .equ foo 42" 22
+chkerr "foo:\\foo:" 22
 chkoom

tools/tests/zasm/test7.asm

@@ -0,0 +1,5 @@
+; It's fine to declare the same constant twice. Only the first value is
+; kept
+.equ	FOO	42
+.equ	FOO	22
+ld	a, FOO

tools/tests/zasm/test7.asm.expected

@@ -0,0 +1 @@
+>*