configs: am335x_evm: Enable FASTBOOT based on kconfig

When CONFIG_CMD_FASTBOOT or CONFIG_USB_FUNCTION_FASTBOOT are defined
in am335x_evm.h, a dependency on g_dnl.c is created. This in turn
creates a dependency on having USB gadget enabled.
As a result we can't create configs with USB gadget disabled.

Since these CONFIG_ variables are now part of kconfig, move them to
the board defconfigs, and out of am335x_evm.h. This both preserves
current defaults, and allows creating configs with USB gadget off.

Signed-off-by: Alexandru Gagniuc <alex.g@adaptrum.com>
Reviewed-by: Tom Rini <trini@konsulko.com>