cmd: Add command to display or save Linux PStore dumps

This patch adds a new pstore command allowing to display or save ramoops
logs (oops, panic, console, ftrace and user) generated by a previous
kernel crash.
PStore parameters can be set in U-Boot configuration file, or at run-time
using "pstore set" command. Records size should be the same as the ones
used by kernel, and should be a power of 2.
This command allows:
- to display uncompressed logs
- to save compressed or uncompressed logs, compressed logs are saved as a
  compressed stream, it may need some work to be able to decompress it,
  e.g. adding a fake header:
  "printf "\x1f\x8b\x08\x00\x00\x00\x00\x00\x00\x00" |
  cat - dmesg-ramoops-0.enc.z | gzip -dc"
- ECC part is not used to check memory corruption
- only 1st FTrace log is displayed or saved

Signed-off-by: Frédéric Danis <frederic.danis@collabora.com>
[trini: Minor updates for current design, correct spacing in rST]
Signed-off-by: Tom Rini <trini@konsulko.com>
Cc: Heinrich Schuchardt <xypron.glpk@gmx.de>
Cc: Wolfgang Denk <wd@denx.de>
Cc: Heiko Schocher <hs@denx.de>