env: Drop the ENTRY typedef

U-Boot is not supposed to use typedef for structs anymore. Also this name
is the same as the ENTRY() macro used in assembler files, and 'entry'
itself is widely used in U-Boot (>8k matches).

Drop the typedef and rename the struct to env_entry to reduce confusion.

Signed-off-by: Simon Glass <sjg@chromium.org>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>