image.bbclass: put .env files to ${STAGING_DIR}/imgdata/

image.bbclass: put .env files to ${STAGING_DIR}/imgdata/

As STAGING_DIR_TARGET started to point to a recipe specific
sysroot wic is not able to add .env files when .wks file refers
to multiple rootfs recipes.

Used STAGING_DIR instead of STAGING_DIR_TARGET to make the
directory with .env files the same for all recipes.

(From OE-Core rev: 3797cfd7473d3f9b7c0d999dcf9cd9608c8c7c6c)

Signed-off-by: Ed Bartosh <ed.bartosh@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>