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/espresso7420_defconfig b/configs/espresso7420_defconfig
index 9128fa0..34e5b4e 100644
--- a/configs/espresso7420_defconfig
+++ b/configs/espresso7420_defconfig
@@ -20,6 +20,7 @@
 # CONFIG_SYS_LONGHELP is not set
 CONFIG_SYS_PROMPT="ESPRESSO7420 # "
 CONFIG_SYS_PBSIZE=1024
+CONFIG_SYS_BOOTM_LEN=0x800000
 # CONFIG_CMD_SETEXPR is not set
 CONFIG_SYS_RELOC_GD_ENV_ADDR=y
 # CONFIG_MMC is not set