Add procedure for searching RSDT Pointer (RSDP)

The newly added function only searchs the "RSD PTR "-signature in the
Main BIOS Area, not in the *Extended* BIOS Area. That should be fixed,
but right now, it's of little relevance.

i386/legacy-boot/src-asm/drivers/acpi/search-rsdp.asm

@@ -0,0 +1,49 @@
+
+MAIN_BIOS_AREA_START equ 0xe0000
+
+; Arguments:
+;   -- None --
+acpi_search_rsdp:
+.prolog:
+    push        esi
+    sub         esp,                    8
+    mov         esi,                    esp
+
+    mov         [esi],                  dword 0
+    mov         [esi + 4],              ecx
+
+.search_loop_header:
+    mov         ecx,                    MAIN_BIOS_AREA_START
+
+.search_loop:
+    push        ebp
+    mov         ebp,                    esp
+    push        ecx
+    push        dword .signature
+    push        dword 8
+    call        mem_equal
+    mov         esp,                    ebp
+    pop         ebp
+
+    cmp         ax,                     0
+    jne         .rsdp_found
+
+    add         ecx,                    16
+    cmp         ecx,                    0x100000
+    jb          .search_loop
+    
+    ; If no "RSD PTR "-signature could be found
+    xor         ecx,                    ecx
+
+.rsdp_found:
+    mov         eax, ecx
+
+.epilog:
+    mov         ecx,                    [esi + 4]
+    add         esp,                    8
+    pop         esi
+
+    ret
+
+.signature:
+    db "RSD PTR "