u-boot-imx: Fix the build and GIT repository location
1. u-boot-imx build was broken after making the switch to open source
u-boot. Added UBOOT_MACHINE defines since the u-boot config names
are different from that used in the denx GIT repository. Also added
defines UBOOT_MAKE_TARGET & UBOOT_SUFFIX given the older version of
the u-boot code used.
2. Added 'HOSTSTRIP=true' to EXTRA_OEMAKE for build to succeed.
3. Use git.freescale.com to get the u-boot source code.
