linker: add custom align size to reduce alignment memory wasting

when enable CONFIG_CUSTOM_SECTION_ALIGN, it need less alignment
memory for image rom region. But that needs carefully configure
MPU region and sub-regions(ARMv7-M) to cover this feature.

Fixes: zephyrproject-rtos#17337.

Signed-off-by: Wentong Wu <wentong.wu@intel.com>

arch/arm/core/cortex_m/mpu/Kconfig

@@ -44,4 +44,23 @@ config MPU_ALLOW_FLASH_WRITE
 	help
 	  Enable this to allow MPU RWX access to flash memory
 
+config CUSTOM_SECTION_ALIGN
+	bool "Custom Section Align"
+	depends on ARM_MPU
+	help
+	  MPU_REQUIRES_POWER_OF_TWO_ALIGNMENT(ARMv7-M) sometimes cause memory
+	  wasting in linker scripts defined memory sections. Use this symbol
+	  to guarantee user custom section align size to avoid more memory used
+	  for respect alignment. But that needs carefully configure MPU region
+	  and sub-regions(ARMv7-M) to cover this feature.
+
+config CUSTOM_SECTION_MIN_ALIGN_SIZE
+	int "Custom Section Align Size"
+	default 32
+	help
+	  Custom algin size of memory section in linker scripts. Usually
+	  it should consume less alignment memory. Alougth this alignment
+	  size is configured by users, it must also respect the power of
+	  two regulation if hardware requires.
+
 endif # CPU_HAS_MPU

include/arch/arm/cortex_m/scripts/linker.ld

@@ -73,6 +73,9 @@
 	#define RAM_ADDR CONFIG_SRAM_BASE_ADDRESS
 #endif
 
+#if defined(CONFIG_CUSTOM_SECTION_ALIGN)
+_region_min_align = CONFIG_CUSTOM_SECTION_MIN_ALIGN_SIZE;
+#else
 /* Set alignment to CONFIG_ARM_MPU_REGION_MIN_ALIGN_AND_SIZE
  * to make linker section alignment comply with MPU granularity.
  */
@@ -82,8 +85,9 @@ _region_min_align = CONFIG_ARM_MPU_REGION_MIN_ALIGN_AND_SIZE;
 /* If building without MPU support, use default 4-byte alignment. */
 _region_min_align = 4;
 #endif
+#endif
 
-#if defined(CONFIG_MPU_REQUIRES_POWER_OF_TWO_ALIGNMENT)
+#if !defined(CONFIG_CUSTOM_SECTION_ALIGN) && defined(CONFIG_MPU_REQUIRES_POWER_OF_TWO_ALIGNMENT)
 #define MPU_ALIGN(region_size) \
     . = ALIGN(_region_min_align); \
     . = ALIGN( 1 << LOG2CEIL(region_size))