U-Boot: ARM: Add a linux entry point to asm shell code

While the NetBSD entry point is handy as we can use a single uImage
with all 3 blobs, it bypasses U-Boot's own patching of the FDT since
it's not visible to it, so we won't get the RAM size and other things
through it.

src/system/boot/platform/u-boot/arch/arm/shell.S

@@ -30,6 +30,14 @@ SYMBOL(_start_netbsd):
 	b	_start_common
 SYMBOL_END(_start_netbsd)
 
+/*
+ * called from bootm with linux loader compatible args
+ */
+SYMBOL(_start_linux):
+	mov	r4,#2
+	b	_start_common
+SYMBOL_END(_start_linux)
+
 
 
 SYMBOL(_start_common):
@@ -63,6 +71,8 @@ SYMBOL(_start_common):
 	beq	start_raw
 	cmp	r4,#1
 	beq	start_netbsd
+	cmp	r4,#2
+	beq	start_linux
 	mov	pc,lr
 SYMBOL_END(_start_common)