bootm: Add a Kconfig option for bootm functionality

Create a separate Kconfig option which enables the bootm logic,
separate from the 'bootm' command. This will eventually allow booting
without CMDLINE enabled.

Update boards which disable CMD_BOOTM to disable BOOTM instead, since
CMD_BOOTM now depends on BOOTM

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/configs/xilinx_zynqmp_mini_defconfig b/configs/xilinx_zynqmp_mini_defconfig
index 694fa11..c2977d4 100644
--- a/configs/xilinx_zynqmp_mini_defconfig
+++ b/configs/xilinx_zynqmp_mini_defconfig
@@ -13,6 +13,7 @@
 CONFIG_SYS_MEMTEST_START=0x00000000
 CONFIG_SYS_MEMTEST_END=0x00001000
 CONFIG_REMAKE_ELF=y
+# CONFIG_BOOTM is not set
 # CONFIG_LEGACY_IMAGE_FORMAT is not set
 # CONFIG_AUTOBOOT is not set
 # CONFIG_DISPLAY_CPUINFO is not set
@@ -27,7 +28,6 @@
 # CONFIG_CMD_BDI is not set
 # CONFIG_CMD_CONSOLE is not set
 # CONFIG_CMD_BOOTD is not set
-# CONFIG_CMD_BOOTM is not set
 # CONFIG_CMD_BOOTI is not set
 # CONFIG_CMD_ELF is not set
 # CONFIG_CMD_FDT is not set