doc: samsung: Extend E850-96 documentation to be comprehensive

doc: samsung: Extend E850-96 documentation to be comprehensive

Add more bootloading details for E850-96 board. New sections cover next
topics:

  - Hardware configuration of the boot device
  - Flashing and updating the software (multiple methods)
  - Booting with Standard Boot (multiple methods)
  - EFI System Partition structure for E850-96
  - Loadable firmware (LDFW) note
  - Ethernet and USB Host Support

That documents all recently enabled U-Boot features for E850-96, which
can hopefully make U-Boot more useful for the users and developers of
this platform.

Signed-off-by: Sam Protsenko <semen.protsenko@linaro.org>