rockchip: spl: make boot0 hook TPL safe

When building for a TPL/SPL setup (e.g. on the RK3368), we need the
TPL stage to have the extra space for for the 'Rockchip SPL name'
(i.e. 'RK33' word).  Yet, the SPL will start execution at its first
word (i.e. the first word in the SPL binary needs to be a valid
instruction).  To make things a bit more involved, CONFIG_SPL_BUILD
is defined both for the SPL and the TPL stage.

To avoid having to explicitly test for the first stage (TPL, if and
only if TPL and SPL are built, SPL otherwise), this commit modifies
the sequence to repeat the 'b reset' (instead of reserving 4 bytes
of undefined space) at the start of the boot0 hook: if overwritten
(and execution starts at the second word), the first instruction is
still a 'b reset'... if not overwritten, we start on a 'b reset' as
well.

This solution wouldn't even require the check whether we are in the
SPL/TPL build (i.e. CONFIG_SPL_BUILD), but we leave this check in for
documentation purposes.

Signed-off-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>
Reviewed-by: Simon Glass <sjg@chromium.org>

arch/arm/include/asm/arch-rockchip/boot0.h

@@ -1,3 +1,4 @@
+
 /*
  * Copyright 2017 Theobroma Systems Design und Consulting GmbH
  *
@@ -13,7 +14,17 @@
  */
 
 #ifdef CONFIG_SPL_BUILD
-	.space 0x4         /* space for the 'RK33' */
+	/*
+	 * We need to add 4 bytes of space for the 'RK33' at the
+	 * beginning of the executable.	 However, as we want to keep
+	 * this generic and make it applicable to builds that are like
+	 * the RK3368 (TPL needs this, SPL doesn't) or the RK3399 (no
+	 * TPL, but extra space needed in the SPL), we simply repeat
+	 * the 'b reset' with the expectation that the first one will
+	 * be overwritten, if this is the first stage contained in the
+	 * final image created with mkimage)...
+	 */
+	b reset	 /* may be overwritten --- should be 'nop' or a 'b reset' */
 #endif
 	b reset