Convert CONFIG_SYS_BOOTM_LEN to Kconfig

This converts the following to Kconfig:
   CONFIG_SYS_BOOTM_LEN

As part of this, rework error handling in boot/bootm.c so that we pass
the buffer size to handle_decomp_error as CONFIG_SYS_BOOTM_LEN will not
be available to host tools but we do know the size that we passed to
malloc().

Cc: Soeren Moch <smoch@web.de>
Signed-off-by: Tom Rini <trini@konsulko.com>
diff --git a/configs/xilinx_zynqmp_virt_defconfig b/configs/xilinx_zynqmp_virt_defconfig
index 855a1c9..89622d1 100644
--- a/configs/xilinx_zynqmp_virt_defconfig
+++ b/configs/xilinx_zynqmp_virt_defconfig
@@ -55,6 +55,7 @@
 CONFIG_SPL_ATF_NO_PLATFORM_PARAM=y
 CONFIG_SYS_MAXARGS=64
 CONFIG_SYS_PBSIZE=2073
+CONFIG_SYS_BOOTM_LEN=0x6400000
 CONFIG_CMD_BOOTMENU=y
 CONFIG_CMD_THOR_DOWNLOAD=y
 CONFIG_CMD_NVEDIT_EFI=y