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/chromebit_mickey_defconfig b/configs/chromebit_mickey_defconfig
index 862b37d..dad3b36 100644
--- a/configs/chromebit_mickey_defconfig
+++ b/configs/chromebit_mickey_defconfig
@@ -36,6 +36,7 @@
 # CONFIG_SPL_CRC32 is not set
 CONFIG_SPL_SPI_LOAD=y
 CONFIG_SYS_SPI_U_BOOT_OFFS=0x20000
+CONFIG_SYS_BOOTM_LEN=0x4000000
 CONFIG_CMD_GPIO=y
 CONFIG_CMD_GPT=y
 CONFIG_CMD_I2C=y