ARM: fix CONFIG_SPL_MAX_SIZE semantics

Remove SPL-related ASSERT() in arch/arm/cpu/u-boot.lds
as this file is never used for SPL builds.

Rewrite the ASSERT() in arch/arm/cpu/u-boot-spl.lds
to separately test image (text,data,rodata...) size,
BSS size, and full footprint each against its own max,
and make Tegra boards check full footprint.

Also, output section mmutable is not used in SPL builds.
Remove it.

Finally, update README regarding the (now homogeneous)
semantics of CONFIG_SPL_[BSS_]MAX_SIZE and add the new
CONFIG_SPL_MAX_FOOTPRINT macro.

Signed-off-by: Albert ARIBAUD <albert.u.boot@aribaud.net>
Reported-by: Benoît Thébaudeau <benoit.thebaudeau@advansee.com>
diff --git a/README b/README
index 14c83f4..19da9c8 100644
--- a/README
+++ b/README
@@ -2831,8 +2831,18 @@
 		CONFIG_SPL_LDSCRIPT
 		LDSCRIPT for linking the SPL binary.
 
+		CONFIG_SPL_MAX_FOOTPRINT
+		Maximum size in memory allocated to the SPL, BSS included.
+		When defined, the linker checks that the actual memory
+		used by SPL from _start to __bss_end does not exceed it.
+		CONFIG_SPL_MAX_FOOTPRINT and CONFIG_SPL_MAX_BSS_SIZE
+		must not be both defined at the same time.
+
 		CONFIG_SPL_MAX_SIZE
-		Maximum binary size (text, data and rodata) of the SPL binary.
+		Maximum size of the SPL image (text, data, rodata, and
+		linker lists sections), BSS excluded.
+		When defined, the linker checks that the actual size does
+		not exceed it.
 
 		CONFIG_SPL_TEXT_BASE
 		TEXT_BASE for linking the SPL binary.
@@ -2845,7 +2855,11 @@
 		Link address for the BSS within the SPL binary.
 
 		CONFIG_SPL_BSS_MAX_SIZE
-		Maximum binary size of the BSS section of the SPL binary.
+		Maximum size in memory allocated to the SPL BSS.
+		When defined, the linker checks that the actual memory used
+		by SPL from __bss_start to __bss_end does not exceed it.
+		CONFIG_SPL_MAX_FOOTPRINT and CONFIG_SPL_MAX_BSS_SIZE
+		must not be both defined at the same time.
 
 		CONFIG_SPL_STACK
 		Adress of the start of the stack SPL will use