dfu: mmc: file buffer: remove static allocation

For writing files, DFU implementation requires the file buffer
with the len at least of file size. For big files it requires
the same big buffer.

Previously the file buffer was allocated as a static variable,
so it was a part of U-Boot .bss section. For 32MiB len of buffer
we have 32MiB of additional space, required for this section.

The .bss needs to be cleared after the relocation.
This introduces an additional boot delay at every start, but usually
the dfu feature is not required at the standard boot, so the buffer
should be allocated only if required.

This patch removes the static allocation of this buffer,
and alloc it with memalign after first call of function:
- dfu_fill_entity_mmc()
and the buffer is freed on dfu_free_entity() call.

This was tested on Trats2.
A quick test with trace. Boot time from start to main_loop() entry:
- ~888ms - before this change (arch memset enabled for .bss clear)
- ~464ms - after this change

Signed-off-by: Przemyslaw Marczak <p.marczak@samsung.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Cc: Lukasz Majewski <l.majewski@samsung.com>
Cc: Stephen Warren <swarren@nvidia.com>
Cc: Pantelis Antoniou <panto@antoniou-consulting.com>
Cc: Tom Rini <trini@konsulko.com>
Cc: Marek Vasut <marek.vasut@gmail.com>