binman: Don't decompress data while signing

While signing a fit compressed data (i.e. 'blob-ext') is decompressed,
but never compressed again. When compressed data was wrapped in a
section, decompression leads to an error because the outer section had
the original compressed size but the inner entry has the
uncompressed size now.

While singing there is no reason to decompress data. Thus, decompression
should be disabled.

Furthermore, bintools should be collected before loading the data. This
way bintools are available if processing is required on a node.

Signed-off-by: Lukas Funke <lukas.funke@weidmueller.com>
Reviewed-by: Simon Glass <sjg@chromium.org>