samples: big_http_download: Set min_flash to 128K.
samples: big_http_download: Set min_flash to 128K.

Previously there was just "min_ram: 32", which took care about
suitable flash size implicitly. But now we have interesting boards
(e.g. lpcxpresso54114_m0) which tout RAM of 32K and Flash of 64K.
So, become more explicit about Flash requirement for the sample.

Signed-off-by: Paul Sokolovsky <35cb557f011771c94f127d6fef99f804b62203f5@linaro.org>

GiulianoFranchetto/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,explora26/zephyr,finikorg/zephyr,nashif/zephyr,finikorg/zephyr,galak/zephyr,nashif/zephyr,GiulianoFranchetto/zephyr,Vudentz/zephyr,zephyrproject-rtos/zephyr,ldts/zephyr,galak/zephyr,Vudentz/zephyr,finikorg/zephyr,GiulianoFranchetto/zephyr,galak/zephyr,Vudentz/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,explora26/zephyr,nashif/zephyr,Vudentz/zephyr,ldts/zephyr,nashif/zephyr,galak/zephyr,ldts/zephyr,finikorg/zephyr,GiulianoFranchetto/zephyr,ldts/zephyr,zephyrproject-rtos/zephyr,ldts/zephyr,Vudentz/zephyr,zephyrproject-rtos/zephyr,explora26/zephyr,GiulianoFranchetto/zephyr,explora26/zephyr,Vudentz/zephyr,explora26/zephyr,nashif/zephyr