cmake/modules: Make SUBALIGN parameter to zephyr_iterable_sections optional

Most uses of this macro should *not* include SUBALIGN as that can reduce
the alignment requirement for data and cause unaligned access faults in
the processor. Leave support for this in place in case there are
still uses for it.

Signed-off-by: Keith Packard <keithp@keithp.com>

Author: keith-packard

cmake/modules/extensions.cmake

@@ -5288,6 +5288,7 @@ function(zephyr_iterable_section)
   set(options     "ALIGN_WITH_INPUT;NUMERIC")
   set(single_args "GROUP;LMA;NAME;SUBALIGN;VMA")
   set(multi_args  "")
+  set(subalign "")
   set(align_input)
   cmake_parse_arguments(SECTION "${options}" "${single_args}" "${multi_args}" ${ARGN})
 
@@ -5297,10 +5298,8 @@ function(zephyr_iterable_section)
     )
   endif()
 
-  if(NOT DEFINED SECTION_SUBALIGN)
-    message(FATAL_ERROR "zephyr_iterable_section(${ARGV0} ...) missing "
-                        "required argument: SUBALIGN"
-    )
+  if(DEFINED SECTION_SUBALIGN)
+    set(subalign "SUBALIGN ${SECTION_SUBALIGN}")
   endif()
 
   if(SECTION_ALIGN_WITH_INPUT)
@@ -5321,7 +5320,7 @@ function(zephyr_iterable_section)
     NAME ${SECTION_NAME}_area
     GROUP "${SECTION_GROUP}"
     VMA "${SECTION_VMA}" LMA "${SECTION_LMA}"
-    NOINPUT ${align_input} SUBALIGN ${SECTION_SUBALIGN}
+    NOINPUT ${align_input} ${subalign}
   )
   zephyr_linker_section_configure(
     SECTION ${SECTION_NAME}_area