configs: am335x_boneblack_vboot_defconfig: Fix regression by enabling BLK and DM...

configs: am335x_boneblack_vboot_defconfig: Fix regression by enabling BLK and DM support, disable in SPL

This patch adds BLK and DM support for verified boot on TI AM335x
chipsets. The following compile warnings are removed:

===================== WARNING ======================
This board does not use CONFIG_DM_MMC. Please update
the board to use CONFIG_DM_MMC before the v2019.04 release.
Failure to update by the deadline may result in board removal.
See doc/driver-model/MIGRATION.txt for more info.
====================================================
===================== WARNING ======================
This board does not use CONFIG_DM_USB. Please update
the board to use CONFIG_DM_USB before the v2019.07 release.
Failure to update by the deadline may result in board removal.
See doc/driver-model/MIGRATION.txt for more info.
====================================================

BLK and DM_MMC are enabled by default in SPL as well, which is
making the build to break with an overflow(spl image doesn't
fit into SRAM because of size constraints).

  LD      spl/drivers/built-in.o
  LD      spl/u-boot-spl
arm-linux-ld.bfd: u-boot-spl section .u_boot_list will not fit in region .sram
arm-linux-ld.bfd: region .sram overflowed by 116 bytes
make[1]: *** [spl/u-boot-spl] Error 1
make: *** [spl/u-boot-spl] Error 2

For the above reason BLK and DM_MMC is disabled in SPL.
Built and tested on AM335x device (BeagleboneBlack).

Signed-off-by: Suniel Mahesh <sunil.m@techveda.org>