doc: board: ti: k3: Reuse build instructions

Introduce common variables to define a generic build instruction that is
then used in specific board specific description.

Labels are introduced in the evm.rst files to be then reused in variant
board documentation as well.

While at this, drop using ARCH=arm when building u-boot sources. This
practice has been discouraged for some time and can potentially create
problems with Kconfig rules related to aarch64. It's best to avoid
this approach.

Signed-off-by: Nishanth Menon <nm@ti.com>