eal: remove exec-env directory

eal: remove exec-env directory

Only one header file (rte_kni_common.h) was in the sub-directory
	include/exec-env/
This file was installed in a sub-directory of the same name
in the makefile-based build.
Source and install directories are moved as below:

   lib/librte_eal/linux/eal/include/exec-env/
-> lib/librte_eal/linux/eal/include/

   build/include/exec-env/
-> build/include/

The consequence is to have a file hierarchy a bit more flat.

Signed-off-by: Thomas Monjalon <thomas@monjalon.net>
Reviewed-by: David Marchand <david.marchand@redhat.com>
Tested-by: David Marchand <david.marchand@redhat.com>
Acked-by: Ferruh Yigit <ferruh.yigit@intel.com>