fix: cleanup shellcheck warnings for rootfs-image module

[aduskett]

This pull request cleans up several shellcheck warnings, incuding:

• SC2005 useless echo
• SC2166 Preffer || or && instead of -o
• SC2086 Douple quote

Changelog: None
Ticket: None

[mender-test-bot]

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[aduskett]

@mender-test-bot start pipeline --pr mender/1675

[mender-test-bot]

Hello 😺 I created a pipeline for you here: Pipeline-1484360760

Build Configuration Matrix

Key	Value
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BUILD_BEAGLEBONEBLACK	true
BUILD_CLIENT	true
BUILD_QEMUX86_64_BIOS_GRUB	true
BUILD_QEMUX86_64_BIOS_GRUB_GPT	true
BUILD_QEMUX86_64_UEFI_GRUB	true
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BUILD_VEXPRESS_QEMU_UBOOT_UEFI_GRUB	true
CREATE_ARTIFACT_WORKER_REV	master
DEPLOYMENTS_ENTERPRISE_REV	master
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DEVICEAUTH_ENTERPRISE_REV	master
DEVICEAUTH_REV	master
DEVICECONFIG_REV	master
DEVICECONNECT_REV	master
DEVICEMONITOR_REV	master
GENERATE_DELTA_WORKER_REV	master
GUI_REV	master
INTEGRATION_REV	master
INVENTORY_ENTERPRISE_REV	master
INVENTORY_REV	master
IOT_MANAGER_REV	master
MENDER_ARTIFACT_REV	master
MENDER_BINARY_DELTA_REV	master
MENDER_CLI_REV	master
MENDER_CONFIGURE_MODULE_REV	master
MENDER_CONNECT_REV	master
MENDER_CONVERT_REV	master
MENDER_GATEWAY_REV	master
MENDER_REV	pull/1675/head
MENDER_SETUP_REV	master
MENDER_SNAPSHOT_REV	master
MONITOR_CLIENT_REV	master
RUN_BACKEND_INTEGRATION_TESTS	true
RUN_INTEGRATION_TESTS	true
TENANTADM_REV	master
TEST_QEMUX86_64_BIOS_GRUB	true
TEST_QEMUX86_64_BIOS_GRUB_GPT	true
TEST_QEMUX86_64_UEFI_GRUB	true
TEST_VEXPRESS_QEMU	true
TEST_VEXPRESS_QEMU_FLASH	true
TEST_VEXPRESS_QEMU_UBOOT_UEFI_GRUB	true
USERADM_ENTERPRISE_REV	master
USERADM_REV	master
WORKFLOWS_ENTERPRISE_REV	master
WORKFLOWS_REV	master