Call to boot_write_trailer() failing silently

[dillonj-sentient]

The call to boot_write_trailer() was failing silently in my project. This ended up being due to flash_area_align() returning 16. (Writes to internal flash on the STM32U5 must be aligned to quadword addresses.) This is greater than the default value of 8 for BOOT_MAX_ALIGN.
This prevented the trailer information from writing to the secondary slot. This resulted in my swap type always being "temp", despite setting the permanent parameter in the call to boot_set_pending_multi() to 1.

Initially, I was seeing the strange behavior of swap type being temp with no clue as to why this was the case. Defining MCUBOOT_BOOT_MAX_ALIGN 16 in mcuboot_config.h ended up solving this problem. (I was actually trying to understand why my call to boot_set_next() wasn't behaving as I expected.) I recommend a BOOT_LOG_ERR to be added in the case that align > BOOT_MAX_ALIGN so that boot_write_trailer() doesn't silently return -1.