uboot: add option to depend on host-lzop

uboot: add option to depend on host-lzop

Some U-Boot configurations use lzop, which may not be available on the
build machine since it's not a mandatory dependency of Buildroot.

To solve this, a new option BR2_TARGET_UBOOT_NEEDS_LZOP is introduced,
and handled in a similar way to the existing
BR2_TARGET_UBOOT_NEEDS_OPENSSL option.

This fixes the build of the dra7xx_evm U-Boot defconfig as of U-Boot
2018.07, on a build machine without lzop installed.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>