fastboot: getvar: Refactor fastboot_*_get_part_info() usage

Extract fastboot_*_get_part_info() usage for MMC and NAND into
getvar_get_part_info() function, as it will be needed further in other
functions. This way we can avoid code duplication and mess with
preprocessor directives across all points of usage.

Signed-off-by: Sam Protsenko <semen.protsenko@linaro.org>
Reviewed-by: Lukasz Majewski <lukma@denx.de>
Reviewed-by: Igor Opaniuk <igor.opaniuk@toradex.com>