spl: ymodem: Add support for loading gzip compressed uImage

Add support for gunzip-ing gzip-compressed uImages in the SPL Ymodem code.
Loading data over Ymodem can be gruelingly slow, gzip-ing the data can
reduce that aggravating slowness at least slightly (depends on the data,
u-boot.bin compresses to ~1/3 of it's original size on ARM64), hence add
optional support for decompressing gzip-compressed uImages.

Signed-off-by: Marek Vasut <marek.vasut+renesas@gmail.com>
Cc: Tom Rini <trini@konsulko.com>