The files disk_access_usdhc.c, disk_access_spi_sdhc.c,
disk_access_stm32_sdmmc.c, disk_access_ram.c and
disk_access_flash.c are actually drivers for block devices and SD/MMC
controllers. This patch moves this drivers to drivers/disk and
reworks the configuration so that the drivers are selected when
the corresponding node is enabled.
Signed-off-by: Johann Fischer <johann.fischer@nordicsemi.no>