tisdk-image.bbclass: Create SECONDARY_BOOTLOADER_NAME variable
tisdk-image.bbclass: Create SECONDARY_BOOTLOADER_NAME variable

* Am180x-evm secondary bootloader isn't built as part of OE.
* Tisdk-image.bbclass fails since no MLO is copied as part of the am180x-evm
  SDK.
* Use a user defined variable to specify the secondary bootloader and allow the
  copy step to be skipped if secondary bootloader variable is an empty string.

Signed-off-by: Franklin S. Cooper Jr <b8c01a1703dc362919183054a071c1f348818c23@ti.com>
Signed-off-by: Denys Dmytriyenko <d29de71aea38aad3a87d486929cb0aad173ae612@ti.com>

rcn-ee/meta-arago,rcn-ee/meta-arago,rcn-ee/meta-arago,rcn-ee/meta-arago,rcn-ee/meta-arago,MentorEmbedded/meta-arago,MentorEmbedded/meta-arago,MentorEmbedded/meta-arago,MentorEmbedded/meta-arago