pru-software-support: add library for PRU firmware

TI provides a set of headers files and libraries useful in developing
firmware for real-time (PRU) cores embedded in some processors e.g.
AM3358.  This package stages these files for any packages creating
PRU firmware.

Note: As per [1], use commit v4.0.2 to sync with common TI Linux
      versions.

[1] http://e2e.ti.com/support/arm/sitara_arm/f/791/p/552190/2018113#2018113



Signed-off-by: Ash Charles <ash.charles@savoirfairelinux.com>
[Thomas:
 - rename BR2_PACKAGE_PRU_EXAMPLES to BR2_PACKAGE_PRU_SOFTWARE_SUPPORT,
   since the package directory name should match the Config.in option
   for this package
 - use select for BR2_PACKAGE_HOST_TI_CGT_PRU, and therefore add the
   appropriate "depends on BR2_PACKAGE_HOST_TI_CGT_PRU_ARCH_SUPPORTS".]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>