boot: bootutil: Allow ports to size the image hash buffer

[danielriege]

What

Adds MCUBOOT_BOOT_TMPBUF_SZ, letting a port override the size of the buffer
used to read an image in chunks while computing its hash. The default stays at
256 bytes.

Why

BOOT_TMPBUF_SZ determines the chunk size in bootutil_img_hash(), so the
number of flash_area_read() calls over an image is
ceil(image_size / BOOT_TMPBUF_SZ).

On flash that is memory mapped the per-call cost is small. On flash that is not
— QSPI, SPI NOR, eMMC — each read is a bus transaction with a fixed
command/address overhead, so the read count rather than the byte count can
dominate validation time. At the 256 byte default a 1 MiB image is 4096
sequential reads.

The converse also applies: the buffer is statically allocated, so a
RAM-constrained port may want it smaller than 256.

Why not just pass a different tmp_buf_sz?

bootutil_img_hash() already takes the buffer and its size as parameters, but
every caller lives in boot/bootutil or in a port:

• boot/bootutil/src/bootutil_loader.c
• boot/bootutil/src/loader.c
• boot/boot_serial/src/boot_serial_encryption.c
• boot/zephyr/single_loader.c, boot/zephyr/firmware_loader.c
• boot/mynewt/src/single_loader.c

A port that uses the in-tree loaders therefore has no way to reach it short of
forking those files. bootutil_priv.h already includes
mcuboot_config/mcuboot_config.h above the definition, so a guard is all that
is needed.

Design

Mirrors the existing MCUBOOT_BOOT_MAX_ALIGN -> BOOT_MAX_ALIGN pattern in
boot/bootutil/include/bootutil/bootutil_public.h — same #ifdef /
_Static_assert / #else default shape, and the option name is the existing
internal name with the usual MCUBOOT_ prefix, so no new vocabulary is
introduced.

Happy to rename it, or to add a corresponding Zephyr Kconfig option, if
maintainers prefer — note that this guard is a prerequisite for such a Kconfig
option working at all.

Impact

• No behavioural change in any existing configuration; the default is unchanged
  and the new macro is defined nowhere in tree.
• The simulator does not set it, so simulator behaviour is unchanged.
• Documented in samples/mcuboot_config/mcuboot_config.template.h.
• Release note stub included.