base-files: rename 'sdcard' to 'legacy-sdcard'

While an image layout based on MBR and 'bootfs' partition may be easy
to understand for users who are very used to the IBM PC and always have
the option to access the SD card outside of the device (and hence don't
really depend on other recovery methods or dual-boot), in my opinion
it's a dead end for many desirable features on embedded systems,
especially when managed remotely (and hence without an easy option to
access the SD card using another device in case things go wrong, for
example).

Let me explain:

* using a MSDOS/VFAT filesystem to store kernel(s) is problematic, as a
  single corruption of the bootfs can render the system into a state
  that it no longer boots at all. This makes dual-boot useless, or at
  least very tedious to setup with then 2 independent boot partitions
  to avoid the single point of failure on a "hot" block (the FAT index
  of the boot partition, written every time a file is changed in
  bootfs). And well: most targets even store the bootloader environment
  in a file in that very same FAT filesystem, hence it cannot be used
  to script a reliable dual-boot method (as loading the environment
  itself will already fail if the filesystem is corrupted).

* loading the kernel uImage from bootfs and using rootfs inside an
  additional partition means the bootloader can only validate the
  kernel -- if rootfs is broken or corrupted, this can lead to a reboot
  loop, which is often a quite costly thing to happen in terms of
  hardware lifetime.

* imitating MBR-boot behavior with a FAT-formatted bootfs partition
  (like IBM PC in the 80s and 90s) is just one of many choices on
  embedded targets. There are much better options with modern U-Boot
  (which is what we use and build from source for all targets booting
  off SD cards), see examples in mediatek/mt7622 and mediatek/mt7623.

Hence rename the 'sdcard' feature to 'legacy-sdcard', and prefix
functions with 'legacy_sdcard_' instead of 'sdcard_'.

Tested-by: Stijn Tintel <stijn@linux-ipv6.be>
Signed-off-by: Daniel Golle <daniel@makrotopia.org>

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