env: add CONFIG_ENV_SECT_SIZE_AUTO

env: add CONFIG_ENV_SECT_SIZE_AUTO

This is roughly the U-Boot side equivalent to commit
e282c422e0 (tools: fw_env: use erasesize from MEMGETINFO ioctl). The
motivation is the case where one has a board with several revisions,
where the SPI flashes have different erase sizes.

In our case, we have an 8K environment, and the flashes have erase
sizes of 4K (newer boards) and 64K (older boards). Currently, we must
set CONFIG_ENV_SECT_SIZE to 64K to make the code work on the older
boards, but for the newer ones, that ends up wasting quite a bit of
time reading/erasing/restoring the last 56K.

At first, I wanted to allow setting CONFIG_ENV_SECT_SIZE to 0 to mean
"use the erase size the chip reports", but that config
option is used in a number of preprocessor conditionals, and shared
between ENV_IS_IN_FLASH and ENV_IS_IN_SPI_FLASH.

So instead, introduce a new boolean config option, which for now can
only be used with ENV_IS_IN_SPI_FLASH. If left off, there's no change
in behaviour.

The only slightly annoying detail is that, when selected, the compiler
is apparently not smart enough to see that the the saved_size and
saved_offset variables are only used under the same "if (sect_size >
CONFIG_ENV_SIZE)" condition as where they are computed, so we need to
initialize them to 0 to avoid "may be used uninitialized" warnings.

On our newer boards with the 4K erase size, saving the environment now
takes 0.080 seconds instead of 0.53 seconds, which directly translates
to that much faster boot time since our logic always causes the
environment to be written during boot.

Signed-off-by: Rasmus Villemoes <rasmus.villemoes@prevas.dk>