misc: less useless comments; document utility functions; follow the AAPCS calling convention

string.S

@@ -0,0 +1,25 @@
+/*
+    Current implementation only counts the bytes instead of characters.
+
+    @args:
+        r1 => pointer to the string
+    @returns:
+        r0 => string length
+    @clobbers:
+        r2
+*/
+strlen:
+    mov r0, #0
+1:
+    ldrb r2, [r1, r0]
+    cmp r2, #0
+    bxle lr
+
+    add r0, r0, #1
+    b 1b
+
+/*
+    @input r1, r2 - pointers to the strings to compare
+    @return r0 - 0 if equal, -1 if a < b, 1 if a > b
+*/
+strcmp: @ TODO: implement