kconfig: Fix wrong partition size by changing type of FLASH_LOAD_SIZE

FLASH_LOAD_SIZE gets its default from the $(dt_chosen_reg_size)
function, which is defined in scripts/kconfig/kconfigfunctions.py.
$(dt_chosen_reg_size) returns a decimal value rather than a hex value.

Previously, FLASH_LOAD_SIZE was declared as hex, which made Kconfiglib
automatically prepend 0x to its value (the same logic is in the C
Kconfig tools). This gave an incorrect size in .config, e.g.
CONFIG_FLASH_LOAD_SIZE=0x374784 where CONFIG_FLASH_LOAD_SIZE=374784 was
intended.

Fix it by changing the type of FLASH_LOAD_SIZE to int. All other symbols
that use $(dt_node_reg_size) and $(dt_chosen_reg_size) are int too
(FLASH_SIZE and SRAM_SIZE, plus BOOTLOADER_SRAM_SIZE is also int).

This has some subtle breakage potential in that FLASH_LOAD_SIZE has a
prompt (is user-configurable in the menuconfig or by changing .config
files). Nothing seems to directly assign in Zephyr at least.

Fixes: #19877

Signed-off-by: Ulf Magnusson <Ulf.Magnusson@nordicsemi.no>

Kconfig.zephyr

@@ -111,7 +111,7 @@ config FLASH_LOAD_OFFSET
 	  If unsure, leave at the default value 0.
 
 config FLASH_LOAD_SIZE
-	hex "Kernel load size"
+	int "Kernel load size"
 	default $(dt_chosen_reg_size,$(DT_CHOSEN_Z_CODE_PARTITION)) if USE_CODE_PARTITION
 	default 0
 	depends on HAS_FLASH_LOAD_OFFSET