crypto/fsl: Make function names consistent for blob encapsulation/decapsulation.

This patch does the following:

1. The function names for encapsulation and decapsulation
were inconsitent in freescale's implementation and cmd_blob file.
This patch corrects the issues.
2. The function protopye is also modified to change the length parameter
from u8 to u32 to allow encapsulation and decapsulation of larger images.
3. Modified the description of km paramter in the command usage for better
readability.

Signed-off-by: Gaurav Rana <gaurav.rana@freescale.com>
Reviewed-by: Ruchika Gupta <ruchika.gupta@freescale.com>
Reviewed-by: York Sun <yorksun@freescale.com>